Food decay scales to ambient temperature in a realistic(*) fashion

* This is assuming the values for spoils are realistic
* Everything decays at the same rate as before in 65 degree weather,
  decay rate doubles or halves every 18 degrees F, to virtually nil
  at freezing, to 6 times the rate at 105 F. Don't leave meat out in
  a hot summer day. The scaling is more or less in line with reality ( http://channel.nationalgeographic.com/channel/doomsday-preppers/articles/food-preservation/ )
* Car fridges result in a static 20% normal at 65 degree rate, as before. This can be improved upon later.
* Instead of fiddling with item bday, this uses 'rot' and 'last_rot_check'
  variables to maintain a steadily increasing amount of spoilage.
* If/when environmental zones are added, this will be able to vary
  based on submap temperature and shade
* Old code that's a veteran of a few long running games, adjusted;
** To work with current refrigeration
** This does not use a per-hour get_rot_since cache, however that has been written and can be added to reduce performance hit (which isn't noticable thusfar)