supervise-daemon: do not spawn a process if we are exiting

This fixes #375 and allows us to not add another level of indentation in
the supervisor loop.

src/rc/supervise-daemon.c

@@ -648,6 +648,8 @@ static void supervisor(char *exec, char **argv)
 			ts.tv_sec = respawn_delay;
 			ts.tv_nsec = 0;
 			nanosleep(&ts, NULL);
+			if (exiting)
+				continue;
 			child_pid = fork();
 			if (child_pid == -1) {
 				syslog(LOG_ERR, "%s: fork: %s", applet, strerror(errno));