cmd: Fix up checkpatch warnings

Clean up contributing and coding stlye docs

checkpatch emits two warnings of type:

    CHECK: Comparison to NULL could be written "!foo"

Prefer `(!foo)` instead of `(foo == NULL)`.

Do not use comparison to NULL, use !foo
Signed-off-by: Tobin C. Harding <me@tobin.cc>

checkpatch emits warning:

    WARNING: Statements terminations use 1 semicolon

Remove typo'd semicolon.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

checkpatch emits warning:

    WARNING: Block comments use a trailing */ on a separate line

Put trailing */ on a separate line.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

checkpatch emits warning:

    WARNING: unnecessary whitespace before a quoted newline

Remove unnecessary whitespace before a quoted newline
Signed-off-by: Tobin C. Harding <me@tobin.cc>

checkpatch emits warning:

WARNING: static char array declaration should probably be static const char

Use 'const' for static string constant (array of chars).
Signed-off-by: Tobin C. Harding <me@tobin.cc>

checkpatch warns about a bunch of whitespace issues.  Fix the
non-controversial ones.

Fix whitespace issues found by checkpatch.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

checkpatch emites warning:

    WARNING: braces {} are not necessary for single statement blocks

Do not use braces for single statement block.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

checkpatch warns because of function definitions using empty parameter
list.  We should define these functions with 'void' as the parameter.

Use 'void' instead of empty parameter list for function definitions.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

checkpatch emits error:

    ERROR: Macros with complex values should be enclosed in parentheses

Safeguard macro by use of parenthesis.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

Currently for section headings we use fourth level markdown heading
level (####).  We do not have levels two or three.

We can use standard incremental levels for heading adornments i.e

1) =========
2) ##
3) ###
ect.

Since this document is likely referenced by maintainers when guiding new
contributors it can save maintainer time to be able to quickly reference
a section in the coding stlye guide.  If we add numbers to each heading
(like the kernel stlye guide) then maintainers can say:

   Nice patch, please see section 3 of the coding style guide and ...

So, this patch makes two changes

- Use incremental level heading adornments
- Add a number to each section heading
Signed-off-by: Tobin C. Harding <me@tobin.cc>

Heading uses only 3 level header (###) but the rest of the file uses
four (####).  We should be uniform.

Use uniform heading level in line with the rest of the file.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

We explicitly state that LXC uses coding style based on Linux kernel
style.  It is redundant to then document obvious, and well known, kernel
style rules.  Identifier names certainly fall into this category as does
usage of braces.

Remove sections implied by 'kernel style'.  Naming conventions and brace
placement conventions.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

Current explanation of rules around usage of 'extern' are overly
verbose.  It is not necessary to state that functions should be declared
in header files, the compiler already enforces this.  These rules are
simple, they are better described with simple statements.  An example is
not necessary for such simple rules and serves only to make the document
longer.

Use two simple statements describing the rules that govern function
declaration and the usage of the 'extern' keyword.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

Fix minor grammatical issue.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

Currently the coding style guide does not mention that we use kernel
coding style as a base style for LXC.  We have just linked to
CODING_STLYE.md from CONTRIBUTING (which mentions use of kernel coding
style).  We can increase documentation congruence and completeness by
mentioning kernel coding style guide in the introduction to our style
guide.

Add heading and introduction to coding style guide informing readers
that we follow kernel coding style as a base before explaining our style
additions.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

Currently the 'Coding Style' section mentions only the kernel coding
style.  We have additions on top on this outlined in CODING_STYLE.md.
We should direct readers to this document as well as the kernel docs.

Direct readers to CODING_STLYE.md in the 'Coding Style' section.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

Currently we link to a URL for v4.10 of the kernel docs.  Since we
already mention the kernel tree we should link to the _latest_ kernel
docs online instead of a fixed past version.

Link to latest online kernel docs tracking the mainline instead of past
fixed version.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

Kernel coding style guide filename is stale, this file has been renamed
in the kernel tree.  While this file still exists we should use the new
filename.

Update reference to kernel coding style guide to use the new file name.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

log: add logging tools for commands; lxc-usernsexec: cleanup and bugfixes

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Add a thread-safe and uniform way to retrieve errno values in programs that are
shipped as part of LXC but are not expected to have access to the logging
system.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Add a thread-safe and uniform way to retrieve errno values in programs that are
shipped as part of LXC but are not expected to have access to the logging
system.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Checkpatch

usernsexec: Make err out vebose for unshare error

checkpatch.pl emits error

    ERROR: do not use assignment in if condition

Move assignment out of if statement.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

checkpatch.pl emits a bunch of warnings about various whitespace
issues.  Fix all these as a single patch since they are all whitespace
only changes.

Fix whitespace issues found by checkpatch.pl
Signed-off-by: Tobin C. Harding <me@tobin.cc>

checkpatch.pl emits error

    ERROR: do not initialise statics to 0

Do not initialise statics to 0.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

checkpatch.pl emits error

    ERROR: do not use assignment in if condition

Correctly indent switch statement.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

Currently if lxc-usernsexec is run on a kernel without user namespaces
enabled the error message is

	unshare: Invalid argument
	read pipe: Success

This error message 'Invalid argument' does not point at the root cause
of the error.  We can help the user out by giving a more detailed error
message and also not using perror() if errno==0.

Improve error message by
 - Printing unshare flags
 - Printing suggested cause of failure (user namespace not enabled)
 - Print error message with fprintf() if errno==0 (EOF)
Signed-off-by: Tobin C. Harding <me@tobin.cc>

doc: Fix minor grammatical errors

Current we have a few minor grammatical errors in the documentation for
lxc container configuration.

Fix minor grammatical errors.
Signed-off-by: Tobin C. Harding <me@tobin.cc>

state: s/sleep()/nanosleep()/

sleep() is not thread-safe but nanosleep() is. Since no resources are allocated
in lxc_wait() it is safe to call nanosleep() without cancellation handlers.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

lxccontainer: minor fixes

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

lxccontainer: improve file locking

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>