Substantial re-engineering for the foundations of hidden characters.

R_SkinUnlock defines the circumstances under which a skin is available. For simplicty's sake, I've currently bound it to an S_SKIN variable so I can toggle it easily, but it WILL be replaced with a hook into the savegame system at some point.
* Currently has three tiers of unlock - freebie (forceskin or modeattacking via a loaded replay), Ringslinger Only, and SP/Coop and Ringslinger.
* I don't know anything about netcode so I basically decided to make R_SkinUnlock relevant only under local circumstances, try as hard as possible to stop bad skin info from getting sent to the server, and then admit defeat once the server has the information. If this is a bad choice, please discipline me and show me how to fix it.
* Character Select now checks for whether the character is hidden or not on menu load and does/undoes it based on that info, but will never touch one disabled via SOC. I also used this opportunity to optimise, checking for/filling out charsel pictures instead of doing it later. (It now also includes special casing for a select screen with zero characters!)
* Mode Attack now hides hidden characters in its character select based on SP rules.

Things that still need to be done:
* ForceSkin_OnChange. Is there a graceful way to handle this?
* No obvious skin name conflicts. Add a salt to the names of hidden skins, and then remove it when they're unhidden?
* The gap between Knuckles' skin number and the first custom character anybody adds will be way too obvious. A seperate hidden skin numbering system? Start at 32 and count up from there? There's a few ways...