Add a hook, create_request. This hook allows modules to modify

Add a hook, create_request.  This hook allows modules to modify
a request while it is being created.  This hook is called for all
request_rec's, main request, sub request, and internal redirect.
When this hook is called, the the r->main, r->prev, r->next
pointers have been set, so modules can determine what kind of
request this is.

Currently, this is only used by the core module, but protocol modules
are going to need to have the ability to affect the request while it is
being read.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88532 13f79535-47bb-0310-9956-ffa450edef68