cgroup: Continue unit reset if cgroup is busy

When part of the cgroup hierarchy cannot be deleted (e.g. because there
are still processes in it), do not exit unit_prune_cgroup early, but
continue so that u->cgroup_realized is reset.

Log the known case of non-empty cgroups at debug level and other errors
at warning level.

Fixes https://github.com/systemd/systemd/issues/12386

(cherry picked from commit 0219b352)