Backport part of r216156 and very minor bits of r218978;

  Collapse the handling of Content-Length / Transfer-Encoding header
  values into the headers loop saving a bit of cpu and making the
  results clearer, handle the (r->main) subrequest case a bit more 
  efficiently as well, cutting out some duplicate tests, and a mild
  whitespace issue in the If-Unmodified-Since test line.

  Fixes a subrequest bug where subrequests *still* would attempt to
  read Transfer-Encoding: chunked request bodies.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/proxy-reqbody-2.0.x@230711 13f79535-47bb-0310-9956-ffa450edef68