This requires us to check that at cgfsng_ops_init, rather than cgfs_init. Cache the hierarchy and cgroup.use info globally rather than putting it into the per-container info, as cgmanager does. This is ok as both cgroup.use and the list of usable hierarchies are in fact global to a lxc run. Closes #952 Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>