Add recursive cgroup_delete_cgroup_ext.

Add recursive cgroup_delete_cgroup_ext.

Changelog:
- add braces to cgroup_delete_cgroup_ext to improve readability
- use enum instead of macro to define flags

The new function allows to recursively delete whole tree of groups. There is
new infrastructure created around (cg_delete_cgroup_controller_recursive).

It has inconsistent behavior when root group removal is requested - it removes
all children (if requested), but does not remove the root itself and returns
success value. This allows simple rewrite of cgclear functions to use this
function.

Original cgroup_delete_cgroup now leads to cgroup_delete_cgroup_ext.

Signed-off-by: Jan Safranek <jsafrane@redhat.com>
Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>