Dedicated server replays

This allows dedicated servers to save replays if netdemo_record is set to Auto Save. Since demos are invalid (as far as I could tell) if there are 0 players in them, the server will not record one if 0 players are present at the start of a race. The server will also immediately stop recording and save a demo if the player count becomes 0 on any given tic. G_BeginRecording had to be moved from Got_mapcmd to P_SetupLevel because dedicated servers don't ever run Got_mapcmd. This is a change that might have other effects I'm unaware of. I haven't observed anything detrimental though.

Working on this also brought into mind a question. Why doesn't a dedicated server start a new race when the first player enters?