This makes simplifying assumptions:  all usable cgroups must be
mounted under /sys/fs/cgroup/controller or /sys/fs/cgroup/contr1,contr2.

Currently this will only work with cgroup namespaces, because
lxc.mount.auto = cgroup is not implemented.  So cgfsng_ops_init()
returns NULL if cgroup namespaces are not enabled.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>