If rewrite rules are used within a <Proxy *></Proxy> block mod_rewrite
unconditionally sets r->proxyreq = PROXYREQ_REVERSE which causes
modules/proxy/proxy_http.c to add the following headers to all regular,
i.e. forward proxy requests:

X-Forwarded-For: <local-client-ip>
X-Forwarded-Host: <local-client-hostname>
X-Forwarded-Server: <local-proxy-hostname>

To prevent this privacy leak mod_rewrite must not change the value of
r->proxyreq if it already has the value of PROXYREQ_PROXY.

Note that this is a very valid scenario as mod_rewrite can be used
e.g. to deny certain buggy user agents access to the proxy.

Patch follows.