core/cgroup: allow to set cgroup path for frozen unit

core/cgroup: allow to set cgroup path for frozen unit

Otherwise, after 'systemctl daemon-reload' or 'daemon-reexec', frozen
units cannot gain cgroup paths and we cannot operate anything on them,
especially, we cannot thaw or stop them.
```
Aug 12 16:26:09 systemd[1]: wd.service: Job 1278 wd.service/stop finished, result=frozen
Aug 12 16:26:09 systemd[1]: Cannot stop frozen unit wd.service.
Aug 12 16:26:09 systemd[1]: wd.service: Cannot realize cgroup for frozen unit.
Aug 12 16:26:09 systemd[1]: Failed to realize cgroups for queued unit wd.service, ignoring: Device or resource busy
```

Follow-up for 23ac08115af83e3a0a937fa207fc52511aba2ffa.