cgroup-util: Properly handle conditions where cgroup.threads is empty after SIGKILL but processes still remain
After sending a SIGKILL to a process, the process might disappear from
`cgroup.threads` but still show up in `cgroup.procs` and still remains in the
cgroup and cause migrating new processes to `Delegate=yes` cgroups to fail with
`-EBUSY`. This is especially likely for heavyweight processes that consume more
kernel CPU time to clean up.
Fix this by only returning 0 when both `cgroup.threads` and
`cgroup.procs` are empty.
(cherry picked from commit 37f0289bf5f2283c187032f83c33ea955b75f119)