gnulib: use more-standard license notice wording

gnulib: use more-standard license notice wording

* gnulib-tool (func_emit_copyright_notice): When emitting a
license notice into a file, use the standard wording as suggested
by the current information for GNU maintainers, except say "file"
rather than "program".  The new wording gives a license version
number, which addresses an issue raised by Glenn Morris in
<http://lists.gnu.org/archive/html/bug-gnulib/2011-09/msg00397.html>.
* m4/onceonly.m4: Use that same wording here, too.