Adjust player init, NPC init, leveling, etc.

Modified "init_player" so that the player starts with a full array of
heavy items (which thus needn't be checked anymore), made adjustments to
"init_npc", "damage_npc" (modifying experience per kill and per level
up), "get_nth_item_type", "equip_heavy_item", "menu_select_callback",
"menu_get_num_rows_callback", and "draw_cell_contents", removed
"add_current_selection_to_inventory" and "get_num_heavy_items_owned",
and updated the list of NPC types.