copyright years: mass-update every January 1

[Jim Meyering]

Karl Berry just mentioned that it's now considered fine (recommended,
even) to update all copyright lists to include the new year on January 1.
Before, it was recommended not to do that, but rather to update each
list only upon modifying the corresponding file in the course of development.

This is documented:

  > maintain(Copyright Notices), specifically:
  >
  > To update the list of year numbers, ...  It is recommended and
  > simpler to add the new year to all files in the package, and be done
  > with it for the rest of the year.

I prefer to have one change set per year updating all copyright year lists,
rather than having a copyright-list update as part of the first
change of the year for files modified in the normal course of development.

So I did the mass-update in coreutils.