SIGTERM handler: use _exit(2), not exit(3)

exit(3) is not async-signal safe (presumably due to atexit(3) processing),
so use _exit(2) instead. This was clearly an omission from the original
code which even had a comment to the effect that _exit(2) needed to be
used. In all likelihood this never caused any problems as atexit(3)
functionality is not used.

src/daemonitor/700_signal.c

@@ -45,7 +45,7 @@ void sigterm_handler(int signum __attribute__((unused)))
     /* uck -- we can't call exit() because it's not async-signal-safe, so we must call _exit() and
      * clean up manually, with no logging */
     unlink(pidfile_filename);
-    exit(0);
+    _exit(0);
 }