Implement a "graceful-stop" for the prefork MPM (might aswell do the hard one

Implement a "graceful-stop" for the prefork MPM (might aswell do the hard one
first).

General approach is to send SIGUSR1 to all children (which will de-listen, and
exit when finished), and to gather all children as they exit.

We don't use a sleep(timeout) for the timeout implementation, because this
would lead to a rut of defunct children until the timeout had expired.

set_graceful_shutdown stolen from Ken Coar. See <3E84B1EC.3050007@Golux.Com>
(28 Mar 2003).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@240270 13f79535-47bb-0310-9956-ffa450edef68