oprange, show_mission, nameofitem, build_mission_list_type,
unit_map, xdvisible, trdswitchown, ontradingblock, trad, check_trade,
unit_type_name, start_stop_unit, scut, scra, mission, multifire,
perform_mission, fuel, NSC_GENITEM): Replace struct genitem with
struct empobj. Remove genitem.h and create a new file empobj.h.
Replace multiple instances of unions of ef_type structures with
one standard union empobj_storage which is a superset of the individual
instances.
