mod_http: Make RFC 7239 Forwarded opt-in for now to be safe
Supporting both methods at the same time may open to spoofing attacks,
whereby a client sends a Forwarded header that is not stripped by a
reverse proxy, leading Prosody to use that instead of the X-Forwarded-*
headers actually sent by the proxy.
By only supporting one at a time, it can be configured to match what the
proxy uses.
Disabled by default since implementations are sparse and X-Forwarded-*
are everywhere.
