Don't produce food without work

Don't produce food without work

We don't want to starve tiny populations, because that would require
players to move trivial amounts of food after explore and such.
growfood() used to simply grow at least 1f when a sector was about to
starve.  That food is almost never eaten by a tiny population, so we
effectively got some production without work.  Fix by taking away that
free food after people ate, in do_feed().