LAN multiplayer

[cxong]

As part of #36 and after implementing #37 , we can build simple LAN multiplayer by having clients send commands to the server as if they are input devices. A POC of this has already been done so it should be feasible.

One nice feature to have would be a game browser; clients will search for existing servers on the LAN and the server will reply with details. Instructions for doing this:
https://en.wikipedia.org/wiki/Broadcast_address
http://lists.cubik.org/pipermail/enet-discuss/2009-August/001230.html

[DavidKnight247]

For gcw zero it would be great if this was included and compatible with the D-Pad style controller for LAN (and WAN?).

[cxong]

Here's some more details about how to find LAN servers, via UDP broadcast:

http://lists.cubik.org/pipermail/enet-discuss/2009-March/001072.html

Every server binds a UDP info socket (just used for pinging the server)
to the same exact port. Game traffic is done on a different port which
differs per server, only the info socket is the same port value for all
servers. The SO_REUSEADDR option is set on this info socket before the
address bind is done, which allows multiple info sockets to live on the
same ip address and port. SO_REUSEADDR causes any packets sent to the
broadcast address to be delivered to ALL sockets bound to that port.
This allows multiple servers to coexist on the same computer and be
pinged properly, so long as their game traffic is at least on different
ports. The server then replies back with information about which port
its actual game traffic is on and other info back to the client that
pinged it.