hastd: Remove pidfile on exit

Make hastd remove its pidfile before exit after catching a signal or a
stop control message, not just on failure to start.

MFC after:	1 week
Reviewed by:	ngie
Differential Revision:	https://reviews.freebsd.org/D60192

(cherry picked from commit 3f2f6bb8f7)

sbin/hastd/hastd.c

@@ -1174,6 +1174,7 @@ main_loop(void)
 	}
 	terminate_workers();
 	proto_close(cfg->hc_controlconn);
+	(void)pidfile_remove(pfh);
 	exit(EX_OK);
 }