Close the child_exit_event which lives only for the scope

of master_main and was orphaned on each soft or fatal restart.

PR: 40932 
Submitted by: Jeff Robbins <jeffr livedata.com>

Backport: r607677

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.0.x@607679 13f79535-47bb-0310-9956-ffa450edef68