This patch adds a new hook (request_status) that gets ran in proxy_handler

This patch adds a new hook (request_status) that gets ran in proxy_handler
just before the final return.  This gives modules an opportunity to do
something based on the proxy status.

A couple of examples where this is useful:

-You are using a caching module and would rather return stale content rather
than an error to the client if the origin is down.

-you proxy some subrequests (using SSI - mod_include) and do not want SSI
errors when the backend is down. If you would normally return
HTTP_BAD_GATEWAY, you may have a module that serves some other content.

new hook -- so mmn bump.. i made it a major one, hope thats ok

Patch From Brian Akins <Brian.Akins turner.com>

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@219372 13f79535-47bb-0310-9956-ffa450edef68