Automatic mounting: add more ways to mount the cgroup filesystem

Automatic mounting: add more ways to mount the cgroup filesystem

This adds quite a few more ways to mount the cgroup filesystem
automatically:

 - Specify ro/rw/mixed:
       - ro: everything mounted read-only
       - rw: everything mounted read-write
       - mixed: only container's own cgroup is rw, rest ro
                (default)
 - Add cgroup-full that mounts the entire cgroup tree to the
   corresponding directories. ro/rw/mixed also apply here.

Signed-off-by: Christian Seiler <christian@iwakd.de>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>