service: enter SERVICE_STOP_SIGTERM state after getting "STOPPING=1"

SERVICE_STOP would mean we're running the ExecStop command. That's not
the case with "STOPPING=1".
Instead we should enter the same state as if we just told the service
to stop, i.e. SERVICE_STOP_SIGTERM.

This fixes a bug where voluntarily exiting services would hang around in
deactivating state until timeout.