Some minor things to help draw sonicitems to a close.

* "kartvoices" cvar. Possible values "Never", "Tasteful" (default), and "Meme".
* Added a way to move quickly through the credits, rather than skip them entirely (hold spacebar or down arrow).
* Fix a few mistakes in M_ChangeCvar, some of which I introduced and some of which were weird in the first place.
* Tweak the offset of the arrows that let you know you can modify a cvar by pressing left or right (some via a patch.kart change, but others via tweaking the drawing location).