Refactor game/mod cvar defaults

Refactor game/mod cvar defaults

Adds gamegroup defaults.
This avoids the need to specify the same Quake defaults for every Quake
expansion/mod.
Fixes newly-added Quake mods (quake15, ad) having global defaults
instead of Quake defaults.

Moves comments in front of the cvars they refer to, some were unclear.

Removes redundant cvars, they're no longer needed because global
defaults and Quake defaults are correctly restored when switching
gamedirs, see 32c99c7b0a58545b273fa713d81d6bbfb3b8ebe6 and
205931f2f5b7333523ca4f6c3cae7974a8f516c8.

See also: 34e819a626f6a7ff2ad190a8e7d42e38ca8cef0e

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>