Autoequip hotkey in inventory screen

Adds 'z' (default) hotkey to autoequip the current soldier.  Complements the existing 'x' (clear), 'c' (copy), and 'v' (paste) hotkeys as per the discussion at: http://openxcom.org/forum/index.php?topic=2781.0

This PR lightly refactors the existing autoequip code so that it can be called from the InventoryState code.  No functional changes were made to the autoequip code.

The nicest bit about this (I think) is that you can manually equip a soldier with a weapon, then hit 'z', and appropriate ammo will be added to the soldiers backpack/belt.