Apply the item position system to the remaining places.

Use item position instead of invlet everywhere.

The only functions in the player class still using invlets is invlet_to_position.
It return the item position of the item with the given invlet. It is only for the user interface, internally everything uses the item position.
Code that used the invlet with other functions (i_at(char), has_item(char)) will now get the item position and than use that value.

game::drop: use the item-vector as parameter to store the taken off armor. Than drop the items from that vector. This does not require any invlet information at all.

game::unload: don't assign an invlet directly, let the inventory class handle it.

Pickup::pick_up: don't assign an invlet, let the inventory class handle it. Auto-wielding still assigns an invlet.

Rename inventory::position_by_letter to invlet_to_position to be consistent with the function of the same purpose in class player.

Added debug message to player::position_by_letter when called on a npc, npcs should not use the invlet system at all.