config/output: Reset everything before swaybg exec

swaybar and the exec command reset signal masks, signal handlers and
NOFILE limit before exec, but swaybg was missing all that.

Reset it for swaybg as well.

sway/config/output.c

@@ -1061,6 +1061,7 @@ static bool _spawn_swaybg(char **command) {
 		return false;
 	} else if (pid == 0) {
 		restore_nofile_limit();
+		restore_signals();
 		if (!sway_set_cloexec(sockets[1], false)) {
 			_exit(EXIT_FAILURE);
 		}