Provide a function for closing all of the listeners.

Provide a function for closing all of the listeners.

  * This is useful for properly implementing a graceful stop and restart
    where we want child processess to be able to carry on serving a request
    but "de-listen" from a port. So that another instance entirely can be
    started in our place, or to unbind from a "Listen" directive an admin
    has removed from the configuration.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@239710 13f79535-47bb-0310-9956-ffa450edef68