[q2x] cvars

[Nick Trout]

Okay, Lua cvars are working with q2x now. Game seems to play okay and you
can load other levels and use gamemap. Bit messy at the moment (but only in
q_cvars.c) as I had to get the old system working in parallel with the new
one to debug it. There is a single #define which turns this feature on and
off.

I'm leaving all the old code in, either #define or commented out until the
system is all working. Might starting converting over all the commands to
use Lua functions now.

Nick