Bustable Wall and Floor flags.

Separates the ability to bust walls from CA_GLIDEANDCLIMB to its own skinflag: SF_CANBUSTWALLS. Alongside SF_CANBUSTWALLS, this also adds a player flag: PF_CANBUSTFLOORS. While a player has this pflag, coming into contact with any bustable floor will bust it. Both of these flags should be very useful for character creators and Lua scripters alike.

NOTE: If this is merged, be sure to add SF_CANBUSTWALLS to Knuckles' S_SKIN flags, otherwise he wont be able to bust walls.