New build: use a DML instead of replacing DARK.GAM

Rather than overwriting the core gamesys file (DARK.GAM) for each game and being subject to possible breakages with future versions of TFix/NewDark, use NewDark's DML text-based file format to override only the gamesys settings that we need. This does mean that the mod now only works with TFix, but I'm guessing anyone interested in this mod is already using that. I've updated the installation instructions to reflect the new install process.

If you absolutely need the old version of this mod, let me know in the comments here and I'll make it available somehow.