Provide insight on proxy_protocol usage

Currently when connecting via proxy protocol it just replaces the actual source address of the client connection with the address given in the PROXY protocol header.

This is completely transparent to later server layers, but there may be circumstances where one might still want to know that proxy protocol was applied, or even want to be able to identify the actual proxy a connection came through (or at least the last one in the proxy chain).

This could e.g. be accomplished by adding some new status variables like proxy_protocol_used and proxy_protocol_via.

There should be only minimal overhead in populating the two at connect time.