clean up nested cgroups

[TommyCox]

I created an OpenRC init script (OpenRC 0.62.10) for a proprietary daemon that appears to be creating a cgroup for its child process. This prevents cleanup of /sys/fs/cgroup/openrc.daemon because /sys/fs/cgroup/openrc.daemon/child.process also exists. Since the Gentoo Wiki page on OpenRC cgroups seems to expect this, perhaps nested cgroup directories should be cleaned up?

I saw that @navi-desu is reworking cgroups in #820, so perhaps just add this as an edge case to test (if it's even relevant in the rework)?

I worked around the problem by manually overriding stop() to delete the nested cgroup. The default cleanup functions still seem to run in this case and clean up the daemon's cgroup directory as well.

I haven't learned much about cgroups yet, so if my guesses are totally off and you need more specific details let me know.

[navi-desu]

first thought is that the rework won't interact with it directly, so i'll be sure to add a test

note, if you're using default_stop, then instead of overriding stop, you can define stop_post() instead and add the code there