Update check-cvars.sh to only disallow changing extra settings in custom balance...

Update check-cvars.sh to only disallow changing extra settings in custom balance files

Now custom balance files like balance-overkill.cfg don't have to change every single cvar balance-xonotic.cfg does, so that the same defaults don't need to repeated for every single balance file.
The only requirement now is that balance-overkill.cfg can't set any cvars which aren't also set in balance-xonotic.cfg.
Additionally balance files can't set cvars with the wrong prefix (like cl_ or r_), and likewise hud files can't set cvars with prefixes other than hud_ or _hud_.
This doesn't apply to the hud cfg files, since they're compared against _hud_descriptions.cfg, which doesn't set defaults, so they must do it themselves.
Changes weren't made to the cfg files in question yet.