Some threadpool fixes:

- We don't need the listener_blocked flag, just check if the element
  we're adding makes the stack non-empty.
- When we are terminated, return APR_EOF; catch this event in the
  worker thread and the listener thread.
- When shutting down, always signal a potential listener thread.
- Wait to signal the listener thread until after we add the worker
  element to the stack.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94841 13f79535-47bb-0310-9956-ffa450edef68