fix: handle SIGTERM signal. (#871)
fix: handle SIGTERM signal. (#871)

systemd send SIGTERM to all processes when system shutdown. We should
handle this signal, save player's status before shutdown.

Signed-off-by: Kasei Wang <e54822cc6a1b34f23ff25a0d118ed68ce410e86d@kasei.im>
darknessomi/musicbox,kigawas/musicbox,kigawas/musicbox