cgfsng: next generation filesystem-backed cgroup implementation
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.