Added a bit of a fun limitation.

* Made it so (player->availabilities & 1 << skinnum) is only true if skins[skinnum] DOES have an availability method, and always false otherwise.
* This means that setting your availabilities to INT32_MAX is no longer the equivalent of having gotten all skins.
* This means we can detect and kick cheat engine script kiddies who try to fool the server that they can use everything.
* This means availabilities of 0x00 is now valid, so make it all F's for an invalid not-in-game.