Changed the threaded mpm to have child_main join to each of the

Changed the threaded mpm to have child_main join to each of the
worker threads to make sure the kids are all gone before child_main
exits after a signal (cleanup from perform_idle_server_maintenance).
This is an extension of Ryans recent commit to make the child_main
the signal thread.

But then thats what the new entry in the file says...

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88859 13f79535-47bb-0310-9956-ffa450edef68