Based on !915 (merged). Merge that one first.
Adapt the plane movement linedef types to UDMF: 50-62, 66-68, 400-401, 403-407, 411, 428-431. Linedef type 408 is new; it sets the tagged sector's flats without moving the planes. I also took the opportunity to refactor those horrible floor/ceiling movement functions a little bit.