Implement a graceful-stop for the worker MPM. We close our listeners, and then
ask each child process to do the same. We then monitor until all children have
exited.

The change to ap_start_shutdown(void) to ap_start_shutdown(int) may look like
an external API change, but the function is defined static, and used only in
one place.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@241819 13f79535-47bb-0310-9956-ffa450edef68