Compromise on cheats setting modified game

Instead of modifying the game, cheats now set a separate "cheats were used in this session" variable, which returns some of the old behavior.

HOWEVER, cheats will STILL allow spawning / collecting emblems & unlocking unlockables. Cheats will purely prevent saving progress. (It was always frustrating that devmode would make debugging unlockable features harder...)

Lastly, the function to set no-saving was exposed to Lua (`G_SetUsedCheats(silent)`). Just thought it'd be useful for large-scale gamedata-using mods that want to add their own cheat commands.