graceful shutdown (no dropped connections) requires that the listener

go away before the workers...  introduce separate XXX_may_exit flags
for our different categories of threads so that a future fix for
graceful shutdown can terminate them in the right order

rename signal_workers() to signal_threads() and give it a parameter
so it knows whether or not termination should be graceful

this commit doesn't change the behavior in any noticeable way;  the
flags used to tell threads to go away are still set at about the same
time


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94095 13f79535-47bb-0310-9956-ffa450edef68