use systemd restart ability to monitor sheepdog daemon

Restart sheepdog if the service process exits due to an uncaught signal
not specified as a clean exit status.

Signed-off-by: Vladislav Gorbunov <vadikgo@gmail.com>
Reviewed-by: Vasiliy Tolstov <v.tolstov@selfip.ru>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>