Add support for more than 2 players

[AXDOOMER]

Right now, only two players can play. It would be great to have support for up to four players.

[AXDOOMER]

It would be possible to do this and the feature would be added only through the command line.

The current source code only supports two players: a server and a client. There must be big changes so that multiple clients can connect to one server and then the game must run smoothly while all clients are sync.

[AXDOOMER]

Also, implement the checksum (check for demo desync) and the chat. For the chat, we could press F7 and then accept alpha-numeric chars as input. Send them over the network to everyone and display them with a sound so it is noticed. The chars must be converted to a string.