Process early errors via a dummy HTTP/1.1 request as well

Process early errors via a dummy HTTP/1.1 request as well to ensure
that the request gets logged correctly and possible custom error
pages are considered. The previous way of directly sending a HTTP/2
answer with the HTTP status code appropriate for the error is more
efficient, but does not log the request nor sents a possible custom
error page.

* modules/http2/h2.h: Add http_status to h2_request struct and define
  H2_HTTP_STATUS_UNSET.

* modules/http2/h2_request.c(h2_request_create_rec): Check if
  http_status is set for the request and die with the
  status code it contains if set.

* modules/http2/h2_session.c(on_header_cb): Adjust the error condition
  now that we mark early errors via http_status: Only return an error
  if the status is not success and http_status is not H2_HTTP_STATUS_UNSET.

* modules/http2/h2_stream.c(set_error_response): Set http_status
  on the request instead of creating headers for a response and a
  respective brigade.

Github: closes #137


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1881620 13f79535-47bb-0310-9956-ffa450edef68
