Convert AT_{SWOOP,KITE} to AF_{SWOOP,KITE} (MarvinPA).

I can't think of a good way to mention these in describe.cc, and the
monsters that use these already mention the special attacks in their
description, so I'm leaving this out of there for now.
Split knockback from AT_TRAMPLE into a new AF_TRAMPLE.

This makes AT_* purely cosmetic (except AT_REACH_STING, which I have no
idea what to do with in any clean fashion), and allows trample attacks
to possibly have other effects (there could be an AF_KNOCKBACK which
means the monster does not follow along, an AF_COLLIDE which could
combine this with trampling into walls, etc.)