Upped MAXNETNODES

This increases the amount of nodes that can join from the default 16 to MAXPLAYERS+4, which should be more than enough for now, which resolves cases like where full servers would show up as unknown on the MS, or can't send a "Server is full" refuse to the client because no more free nodes. An ideal solution would be to separate server list nodes from player nodes, but that's another can of worms to deal with.