This fixes moving in direction '>' while wielding a pick-axe with
the 'autodig' option On. Attempting to dig assumed size-to-side
activity because u.dz wasn't being set, so the pick-axe was swung at
thin air instead making a pit or turning an existing pit into a hole.
dodown() and doup() should probably call set_move_cmd() instead of
manipulating u.dz directly. But they would also need to suppress
g.move_attempting or the game says "you can't get there from here..."
after successfully digging or plunging into a pit.
75fed4b92b