mpm_winnt: Tweak the listener shutdown code to use a separate event

mpm_winnt: Tweak the listener shutdown code to use a separate event
instead of the global variable (shutdown_in_progress).

This change has two purposes.  First of all, it makes the listener threads
which are blocked waiting for a completion context exit immediately during
shutdown.  Previously, such threads would only check for exit every second.
The second reason for this change is to put the child_main() function in
charge of controlling the listeners life cycle.  Previously, such relation
was circumvented by the fact that the listeners were also waiting for the
global child exit_event.  With the new separate listener_shutdown_event,
only the child_main() function is responsible for shutting down the
listeners, and I think that this makes the code a bit clearer.

All the original behavior, including the special APLOG_DEBUG diagnostic
message when we fail to acquire a free completion context in 1 second,
is kept unchanged.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1801747 13f79535-47bb-0310-9956-ffa450edef68