import: Add GNU importer.

Suggested by Karl Berry <karl@gnu.org>
and Andreas Enge <andreas@enge.fr>.

* guix/import/gnu.scm, guix/scripts/import/gnu.scm: New files.
* Makefile.am (MODULES): Add them.
* guix/scripts/import.scm (importers): Add "gnu".
* guix/gnu-maintenance.scm (gnu-release-archive-types): New procedure.
  (download-tarball): Export.