worker MPM: Fix a problem which could cause httpd processes to

worker MPM: Fix a problem which could cause httpd processes to
remain active after shutdown.

The problem occurred when a scoreboard entry currently
in use by an exiting child process was used for a new child
process.  At that point, the MPM forgot about the exiting
child process, so ap_reclaim_child_processes() wouldn't be
able to forceably terminate it.

(An exiting child process may *never* exit due to a stuck
or long-running request being handled on one of the threads.)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@109510 13f79535-47bb-0310-9956-ffa450edef68