On termination lxc may fail to remove either lxc.cgroup.dir or lxc.cgroup.dir.monitor,
because the monitor process may still be a member of either of these cgroups.
The pivot cgroup should not be a member (subpath) of any other container cgroup (dir).
because only empty cgroups can be removed.
Signed-off-by: Ruben Jenster <r.jenster@drachenfels.de>