lxc: Make help/usage a bit more consistent
lxc: Make help/usage a bit more consistent

 - Make sure that we always exit 0 when passed --help
 - Make sure that when exiting 0, the usage goes to stdout.
 - On error, make sure the usage goes to stderr.

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

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