Implement "de-listening" for graceful restarts with the prefork MPM. With this
change;

  1.) httpd -k graceful sends SIGUSR1 to the parent pid, which in turn
      sends SIGUSR1 to all of the active children,

  2.) Active children each close their copy of listener fd's.

This means that the listening sockets are freed for re-use. In the ordinary
case, this makes no difference. However if for example admin changes "Listen
80" to "Listen 81" in the config, this rev makes port 80 immediately available
(no waiting for the graceful children to die).



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@239711 13f79535-47bb-0310-9956-ffa450edef68