MINOR: mux-fcgi: Rely on client addresses at stream level to set default params

MINOR: mux-fcgi: Rely on client addresses at stream level to set default params

Client source and destination addresses at stream level are now used to emit
SERVER_NAME/SERVER_PORT and REMOTE_ADDR/REMOTE_PORT parameters. For now,
stream-interface addresses are never set. So, thanks to the fallback
mechanism, no changes are expected with this patch. But its purpose is to
rely on addresses at the stream level, when set, instead of those at the
connection level.