mod_rewrite: URI-to-filename rewrites to transparently handle proxy mappings.

Since mod_rewrite works on r->filename and mod_proxy's mapping=servlet|decoded
sets its "proxy:" URL there at pre_translate_name stage (i.e. before
mod_rewrite's translate_name hook), users have to match the full proxy URL in
their RewriteRules to handle proxy mappings, which is not very friendly nor
consistent with how proxy non-mapping requests have to be matched.

Let's use r->filename = r->uri in hook_uri2file() for pre_trans'ed reverse
proxy requests, and restore r->filename to its original value if the request
was finally DECLINED (like in hook_fixup).

But if a proxy mapping gets rewritten to a non-proxy request, clear any
proxy specific r->proxyreq or r->handler so that processing continues
accordingly.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1898509 13f79535-47bb-0310-9956-ffa450edef68
