Win32: First cut at new shutdown/restart logic. The key new feature here is

Win32: First cut at new shutdown/restart logic. The key new feature here is
the use of CancelIo on the listen socket. Threads can only cancel
their own I/O. To enable use of this nifty feature, I needed to make
sure only one thread ever initiated async I/O. So, if the workers
detect that they need more completion contexts (async AcceptExes),
they set the maintenance_event, which directs the control thread to
do the work on behalf of the workers. Most of the other changes are
to support this end.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85030 13f79535-47bb-0310-9956-ffa450edef68