* mandos: Never call .Reset() on a defunct Avahi entry group.
  (AvahiService.remove): Only do group.Reset(), as before.
  (AvahiService.add): Only create new group if necessary.
  (AvahiService.cleanup): Also do group.Free() if a group exists.
  (AvahiService.server_state_changed): Call .cleanup() on errors.
                                       Also log error parameter, if
                                       passed.