Prevent players moving too far/fast when stepping up a staircase

After the player made some horizontal progress, and before stepping up, it's necessary to recalculate the remaining time and distance for the move. Previously the step up made the player travel (up to) nearly the move's full distance, again.

Fixes #2586 (closed)

This will require QC physics or the engine patch https://github.com/DarkPlacesEngine/darkplaces/pull/10