worker MPM:

get MaxRequestsPerChild to work again by allowing the main thread of
a child to be interrupted by one of the other threads in the process

this should get graceful termination to work after encountering one of
the various possible error conditions in the listener and worker threads


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94232 13f79535-47bb-0310-9956-ffa450edef68