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 cb3f7f335a)

build-aux/update-b4-copyright

@@ -73,10 +73,19 @@ while (/($old_re)/gx)
         # Put spaces after commas.
         $year_lines =~ s/, ?/, /g;
 
-        # Do not compress to intervals; for example, do not replace
-        # "2008, 2009, 2010" with "2008-2010".  See the Copyright
-        # Notices section in Information for Maintainers of GNU Software, at:
-        # http://www.gnu.org/prep/maintain/maintain.html#Copyright-Notices
+        # Compress to intervals.
+        $year_lines =~
+          s/
+            (\d{4})
+            (?:
+              (,\ |-)
+              ((??{
+                if    ($2 eq '-') { '\d{4}'; }
+                elsif (!$3)       { $1 + 1;  }
+                else              { $3 + 1;  }
+              }))
+            )+
+          /$1-$3/gx;
 
         # Format within margin.
         my $year_lines_new;