Cleanup the proxy code that creates a request to the origin

Cleanup the proxy code that creates a request to the origin
server.  This change adds an optional hook, which allows modules
to gain control while the request is created if the proxy module
is loaded.  The purpose of this hook is to allow modules to add
input and/or output filters to the request to the origin.  While
I was at it, I made the core use this hook, so that proxy request
creation uses some of the code from the core.  This can still be
greatly improved, but this is a good start.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91486 13f79535-47bb-0310-9956-ffa450edef68