Keybindings.
More work on Interface::minister_food().  Move some stuff to a helper function, Interface::list_farm_crops().
When generating a map, clear out all crops/minerals before adding new ones.  This is important because if we examine a World_map tile, it generates the map.  If we decide not to build a city there, then examine it again, it generates the map again.  If we don't clear out the crops/minerals, the new map will have them added twice.