cgroup-util: Properly handle conditions where cgroup.threads is empty after SIGKILL...

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.