Sinew drop rate bugfix, and ability to spawn copies of charged items

Made it possible to create multiple copies of a charged item on the map, by splitting the "quantity" argument into "quantity" and "charges".

I'm told that the charges argument wasn't actually used yet, but it no longer conflicts with the quantity argument (which WAS an issue - butchered sinew was dropping 1/10th what it should have been).

Since I was touching the code anyway, I also renamed that instance of map::add_item() to map::spawn_item(), per Kevin's suggestion. The method which creates a new item on the map is now map::spawn_item(), and the method which drops an existing item on the map is still map::add_item().