Prevent players moving too far when stepping up

Prevent players moving too far when stepping up

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 https://github.com/DarkPlacesEngine/darkplaces/issues/9
sv_gameplayfix_stepmultipletimes 1: players travel further than their velocity should allow #9

Signed-off-by: bones_was_here <bones_was_here@xa.org.au>