Plug AllowTrace extended|on|off into proxy and http core.

  It still is not 'correct' until REQUEST_CHUNKED_PASS is reimplemented
  and passes some chunk headers, since we aren't echoing the entire
  request.  But it gets me further on testing 1.3 -> 2.0 -> 2.1 -> 2.0 -> 1.3
  proxy behaviors.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@208787 13f79535-47bb-0310-9956-ffa450edef68