maint: fix use of copyright year intervals.

* gnulib: Update.
* bootstrap.conf (gnulib_modules): Update getopt to getopt-gnu
as now recommended in gnulib/NEWS.
* build-aux/update-b4-copyright: Fix.
* cfg.mk (update-copyright-env): Configure update-copyright.
(cherry picked from commit 75ac158b82)

lib/.cvsignore

@@ -64,6 +64,7 @@ stamp-h1
 stdbool.h
 stdbool.in.h
 stdbool_.h
+stddef.in.h
 stdint.h
 stdint.in.h
 stdint_.h

lib/.gitignore

@@ -67,6 +67,7 @@
 /stdbool.h
 /stdbool.in.h
 /stdbool_.h
+/stddef.in.h
 /stdint.h
 /stdint.in.h
 /stdint_.h