UltraRogue: fix inventory collision after save and restore. Inventory letters are based on "identifiers" stored in objects' o_ident field. Identifiers are allocated by get_ident(), which keeps a list of objects that have them, to avoid giving the same identifier to multiple objects. The list is not stored in the savefile, so after restore, get_ident() was not aware of existing identifiers. This resulted in picked-up objects having the same inventory letters as objects restored from the file. The restore code now adds all objects with identifiers to the list.