lxc-init: continue even if we fail to mount /dev/mqueue

The 'lxc-init' (a lightweight init process used by lxc-execute in place
of upstart etc) tries to mount /dev/mqueue during startup. If that fails
(for instance due to missing support for mqueue in kernel) then it
aborts execution and returns -1. This is unreasonable as very few
applications actually need /dev/mqueue.

This similar to what we do with /dev/shm.

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>