Display help information in a consistent format.

Print error messages and help information to stderr. Prefix error
messages with the name of the script (for easier debugging as part
of larger scripts).

Allow help information to be printed as a non-root user.

Fix file mode for lxc-checkconfig.in.
Signed-off-by: David Ward <david.ward@ll.mit.edu>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>