* If a subrequest has a broken backend also set no_cache for the main request

* If a subrequest has a broken backend also set no_cache for the main request
  and ensure that the chunk filter does not sent the last chunk marker in this
  case.

  modules/http/chunk_filter.c: Memorize HTTP_BAD_GATEWAY error buckets that
                               had been seen in filter context to ensure
                               that we do not sent the last chunk marker in
                               this case.
  modules/proxy/proxy_util.c : Set no_cache also for main request if we are
                               a subrequest.

  Thanks to Joe Orton and André Malo for the "invented unique pointer" trick.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@366181 13f79535-47bb-0310-9956-ffa450edef68