api.c: add support for empty cgroup v2 deletion

api.c: add support for empty cgroup v2 deletion

Add support to recognize empty cgroup v2 with no controllers attached to
it in cgroup_delete_cgroup_ext(), that gets called to remove a cgroup.

In the deletion path, cgroup_build_tasks_procs_path() builds the path
of the cgroup for re-assignment/movement of tasks from the cgroup that's
getting deleted, teach it to recognize the empty controller as cgroup
V2.

Fixes: https://github.com/libcgroup/libcgroup/issues/125
Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
TJH: Fix a couple small typos in commit message