fix unload visitable error

[KorGgenT]

Summary

SUMMARY: Bugfixes "Fix unloading ammo belts on map"

Purpose of change

Fixes #35368

Describe the solution

When pressing U it brings up a menu to allow you to unload items from your inventory and from the map. the unload function itself assumes that the items are in your inventory, and constructs an item_location from the player object. This leads to an error with the visitable interface, since the item isn't owned by what it thinks it is. A more thorough fix is to refactor the whole unload funciton into 3 or more functions that do separate things, since at least one of the things the current unload function does is to open a prompt.

The actual fix is to call item_location::obtain() in avatar_action::unload() which is the function that opens the inventory prompt and allows you to pick items that are not in your inventory.

Testing

spawn an ammo belt, drop it next to me, and then press U to unload it.