Food decay scales to ambient temperature in a realistic(*) fashion
* This is assuming the values for spoils arre 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.
* Car fridges result in a static 20% normal at 65 degree rate, as before.
* 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
your base are belong to nowhere
Merge PR #5012.