maint: prepare to use date ranges in copyright notices.

* README (Copyright statements): New section explaining the range
notation.  The GNU coding standards require this explanation.  I
copied ours from coreutils.
* build-aux/update-b4-copyright: Revert 2010-06-17 changes that
disabled Bison's automated use of ranges.
* cfg.mk (update-copyright-env): Likewise.
(cherry picked from commit cb3f7f335aa69ba3e6478fea8d4591f4f6d43bc5)