Stop auto-pickup of 0 weight items if already dropped

[Effigy]

The game doesn't automatically try to pick up most items if you previously dropped them, unless the item has 0 encumbrance. This is an annoyance if you have things like gems dropped in your fortress, but it's especially annoying with the Goedalath Rock since that gives penalties when carried. It would be better to just disable auto-pickup for any item the player previously dropped. I assume this is trivial to change since it already works this way for >0 encumbrance items.