Switch to in-tree Lua 5.3.

Lua is not particularly backwards-compatibility friendly, so it's
better to supply it in-tree to ensure you have the version you want.
Plus, 5.3 gives you new toys that are probably good for Doom scripting,
like integers and bitshifts.
Merge pull request #1 from AlexMax/master

Switch to in-tree Lua 5.3.
Merge branch 'netcode'
Break out client-specific and server-specific functions from n_main to cl_main and sv_main respectively