These changes are to allow caching of subrequests via a quick_handler.

* Change SUBREQ_CORE so that it is a HTTP_HEADER (20) filter instead of a content filter (10)
  this allows subrequests to add content filters properly

* Change subreq handling of 'handle-include' so that it splits/passes the brigade before the subreq
  is created. (This allows quick_handler to push content back from this phase)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92956 13f79535-47bb-0310-9956-ffa450edef68