The debugfs, fusectl, and securityfs may not be mounted inside a
non-init userns.  But mountall hangs waiting for them to be
mounted.  So just pre-mount them using $lxcpath/$name/fstab as
bind mounts, which will prevent mountall from trying to mount
them.

If the kernel doesn't provide them, then the bind mount failure
will be ignored, and mountall in the container will proceed
without the mount since it is 'optional'.  But without these
bind mounts, starting a container inside a user namespace
hangs.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>