Merge r722213,r722081, and r721679 from trunk:

* Avoid sending no answer at all if a custom error page causes an
  AP_FILTER_ERROR.

* allow ap_invoke_handler() to pass-through AP_FILTER_ERROR as if it were
  a reserved status code (OK/DECLINED/SUSPENDED). Prevents ap_die() from seeing a
  500 error when the http header filter has already taken care of the proper
  error response

* To be safe, consume the entire brigade after processing an error bucket in
  the HTTP output filter.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@722642 13f79535-47bb-0310-9956-ffa450edef68