Whooh boy. Probably requires re-review.

* Support for MF_PAPERCOLLISION tmhitthing in P_SlideMove.
* Knockback for wallspikes is much more consistent.
* Optimised away a FixedDiv call when determining the slopetype of a line.
* Changed the position of the wallspike base slightly.
* Made it so NOTHING can step up onto the player outside of other players, essentially killing a bunch of springs-launch-with-players style bugs. (I was able to recreate that issue with the wallspikes until I added this, which is why I did it in this branch.)