Use SPDX license headers

Hi,

I would like to replace those long license lines at the top of all our source files with a respective SPDX license identifier:

;;; GNU Guix --- Functional package management for GNU
;;; Copyright © YYYY Name <mail@example.org>
;;;
;;; This file is not part of GNU Guix.
;;;
;;; GNU Guix is free software; you can redistribute it and/or modify it
;;; under the terms of the GNU General Public License as published by
;;; the Free Software Foundation; either version 3 of the License, or (at
;;; your option) any later version.
;;;
;;; GNU Guix is distributed in the hope that it will be useful, but
;;; WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;; GNU General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.

So it becomes something like this:

;;; SPDX-License-Identifier: GPL-3.0-or-later
;;; This file is part of nonguix (<- do we even need/want this line?)
;;; Copyright © YYYY Name <mail@example.org>

What do you think? @podiki @wordempire @ambrevar @roptat