lxc_*.c: don't exit with -1

In this patch I tried to stick with each file's coding style, however I
think we should probably change that.  Every main() should always not
return and only exit;  they should always return EXIT_SUCCESS or EXIT_FAILURE
with the only exceptions being cases where we are returning a child's
exit status (lxc_execute, lxc_attach, lxc_init).

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
cgmanager: cgm_get: len = 0 is a query for length only

The caller might (like lxc-test-startone does) pass in a valid
(or invalid) buffer and 0 length.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>