- ap_listen.h: interface to Listen, ListenBackLog and SendBufferSize

  directives
- I didn't update mpmt_pthread with this change, it shouldn't be hard to fix.
- maybe this isn't the best way to abstract listen... but it gets a little
  bit of the job done


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@83385 13f79535-47bb-0310-9956-ffa450edef68