Add resource import/export and food production/consumption to City::do_turn().  TODO: consequences for lack of resources (e.g. starvation if we don't have enough food).
After accessing a menu item, call Interface::set_mode(IMODE_VIEW_MAP) so that we're not stuck in menu mode without a menu actually open (requiring the played to hit ESC to do anything).
Include instructions on how to close the minister screen.  Move list_empty_fields a bit.  Include a field for days of food remaining.
In Cuss::minister_food(), format the field output field as a percentage, and colorize the empty fields field.  Populate the days left field with a colorized report on how many days's worth of food remains (or N/A if we're at 0 net food or greater).  Update the list of empty fields when it changes (by adding or removing a crop).