These are in GameSocketShare mode, where instead of ISteamGameServer creating its own
socket to talk to the master server on, it lets the game use its socket to forward messages
back and forth. This prevents us from requiring server ops to open up yet another port
in their firewalls.
the IP address and port should be in host order, i.e 127.0.0.1 == 0x7f000001
These are used when you've elected to multiplex the game server's UDP socket
rather than having the master server updater use its own sockets.
Source games use this to simplify the job of the server admins, so they
don't have to open up more ports on their firewalls.
Call this when a packet that starts with 0xFFFFFFFF comes in. That means
it's for us.
