Dropping items!

* Shield Drop...
	* Whatever you've got orbiting or trailing you, DROP THEM WHERE THEY STAND. (Except for the ghost sink. That one's OK.)
	* Pops your Thunder Shield.
	* Happens upon ANY hit, except for deathpits.
* HUD Drop...
	* Also does the above, except for the Thunder Shield thing.
	* If there's any item left in your item box, pop it out as a little hovering, rotating Minecraft item!
	* You can pick up the Minecraft item by driving over it if your item box is sufficiently empty, or the item which is contained within it is of the same type.
	* Happens upon Size Down and battle elimination.
	* Can also be forced on with `cv_kartdebughuddrop on`!
* Some other random stuff.
	* Fix a bunch of `a->scale = b`'s into `P_SetScale(a, b)` form, for maximum validity.
	* Make K_CleanHnextList and K_UpdateHnextList one function, since they only differed by one continue clause (and the type of their input parameter).
	* Allow shrunken players to pick up item boxes again.
	* Fix MF_NOCLIPTHING. (Gonna pass this fix to vanilla when I get the chance, too.)
	* Break NiGHTS a little through my machinations.