Be more consistent with how we lock kartspeed/kartweight

- The skin values are now locked between 1 and 9.
- kartspeed & kartweight are no longer locked on skin-switch. Combined with the above, this results in no gameplay change, other than the character select showing the proper value.
- Values used for the engine sounds are locked. No longer can overflow into character voices or freeslotted sounds.
- Removed the overzealous weight locking when in offroad.

If we want to ACTUALLY lock speed/weight for Lua too then we should do that in the Lua stuff.