cgroup: update debug controller to print out thread mode information

Update debug controller so that it prints out debug info about thread
mode.

 1) The relationship between proc_cset and threaded_csets are displayed.
 2) The status of being a thread root or threaded cgroup is displayed.

This patch is extracted from Waiman's larger patch.

v2: - Removed [thread root] / [threaded] from debug.cgroup_css_links
      file as the same information is available from cgroup.type.
      Suggested by Waiman.
    - Threaded marking is moved to the previous patch.

Patch-originally-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>