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

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