Add multiplayer support

[AXDOOMER]

Here's a checklist for the multiplayer support. It's the most important feature to add.

• Add deathmatch 3.0 (youtube test video)
• Write the multiplayer code (4 players must be able to play)
  -> The parameter -nodes # starts a server that waits for #-1 of players (the server is one node)
  -> -port # changes the server port to # (default is 2345)
  -> -net ip port could also be used. If no port is specified, then the default is used.
  -> Clients use -join ip port to connect (port is optional, default is 2345)
  -> Game options must be sent to the client(s)
• Register games on master.chocolate-doom.org to help people play over the Internet

[GoodSign2017]

Would make it my first priority right after merge.
Registering games would also be great addition. It also can pay attention to our work.

[AXDOOMER]

I made a quick and dirty hack to Mocha Doom to see how hard it would be to implement Deathmatch 3.0. Maes changed a lot of stuff and it was more difficult than I expected. I had to break the support for normal deathmatch and altdeath.

Test video: https://www.youtube.com/watch?v=coWo3UDb5lg
This is from a demo recorded with another source port that I played back with Mocha.

I found two bugs while doing this and I've added them to my TODO list. I will be fixing them when I have free time (I'm into my university exams).

[Fabrice-leterme]

Once Multiplayer is added, I'll start development on this aswell 👍

Quite sure there's alot of fun stuff that can be added to make the multiplayer experience more fun than ever