Add a PROXYREQ_RESPONSE value for request_rec->proxyreq because it is

Add a PROXYREQ_RESPONSE value for request_rec->proxyreq because it is
possible that there can be different behavior at the protocol level if
request_rec isn't really a request but a response.

This stems from the fact that request bodies must be indicated by
Content-Length or Transfer-Encoding, but response bodies do not.  The
recent change to ap_http_filter to return EOS if there isn't a body broke
proxy.  Therefore, there must be some way for the proxy to indicate that
this is a response.  Accordingly, ap_http_filter can allow the BODY_NONE
iff this is a response.

Since r->proxyreq is set to PROXYREQ_PROXY even for the original request
from the client, that value isn't sufficient.  Hence, the introduction of
PROXYREQ_RESPONSE.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95390 13f79535-47bb-0310-9956-ffa450edef68