Half movement cost of most terrain

This makes movement cost equal to 25 * move_cost/move_cost_mod instead of 50, and resets the numbers so terrain & furniture is generally half what it was before (except picket fences & opened windows which are same as before--150 & 200).

This is a bigger change than expected, but testing the player seems to move the right speed, zombies are still a bit  slower than the player, and I'm still about to get 3-4 free hits on a zombies when kited into bushes or windows (more diagonally).  So I think the balance isn't too bad.  The forum post requesting this (http://smf.cataclysmdda.com/index.php?topic=4696.30) got contentious, but there were 6 people in favor of reducing move cost and 2 against.