Newer glibc versions (that we can't require) allow for an additional
letter 'e' in the fopen mode that will cause the file to be opened with
the O_CLOEXEC flag, so that it will be closed if the program exec()s
away. This is important because if liblxc is used in a multithreaded
program, another thread might want to run a program. This options
prevents the leakage of file descriptors from LXC. This patch adds an
emulation for that that uses the open(2) syscall and fdopen(3). At some
later point in time, it may be dropped against fopen(..., "...e").

This commit also converts all fopen() calls in utils.c (where the
function is added) to fopen_cloexec(). Subsequently, other calls to
fopen() and open() should also be adapted.
Signed-off-by: Christian Seiler <christian@iwakd.de>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>