get the start thread to exit when we have reached termination but

it is still trying to create worker threads

previously, after a non-graceful restart followed by a terminate
you could see a bunch of log messages showing the parent repeatedly
sending SIGTERM and finally SIGKILL to one or more children...
with this change, the sequence of messages should stop very soon

add a comment to start_threads() describing a current problem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94030 13f79535-47bb-0310-9956-ffa450edef68