keyfile: relicense the code under LGPL-2.1+

Historically, keyfile read/write code was part of core, and thus
GPL-2.0+ licensed. Keyfile is the native file format for NetworkManager
connection profiles, and code to handle that should be part of libnm.
This would unlock many interesting features, like tools being able
to import/export connection profiles in the native file format.
However, libnm is LGPL-2.1+ licensed, so this is a problem.

The alternative would be to add a separate, GPL licensed library
(libnm-keyfile.so or libnm-gpl.so). However that also requires a larger
rework, because the current keyfile implementation uses internal API
from libnm-core and it would need to be reworked to only use public
API of libnm.

Relicense the code instead. According to research and "keyfile-history.sh"
script, the following individuals and companies possibly hold copyright
on the code:

  <bgalvani(at)redhat.com>
  <blueowl(at)centrum.cz>
  <daniel(at)gnoutcheff.name>
  <danw(at)redhat.com>
  <dcantrell(at)redhat.com>
  <dcbw(at)redhat.com>
  <evan(at)ebroder.net>
  <fgiudici(at)redhat.com>
  <floe(at)butterbrot.org>
  <j(at)bootlab.org>
  <kmaraas(at)gnome.org>
  <lkundrak(at)v3.sk>
  <luzpaz(at)users.noreply.github.com>
  <martinpitt(at)gnome.org>
  <michael.i.doherty(at)intel.com>
  <pavlix(at)pavlix.net>
  <pmarti(at)warp.es>
  <rafaelff(at)gnome.org>
  <rstrode(at)redhat.com>
  <tambet(at)gmail.com>
  <tgraf(at)redhat.com>
  <thaller(at)redhat.com>
  <walters(at)verbum.org>
  <yurchor(at)ukr.net>
  Intel Corporation
  Novell, Inc.
  Red Hat, Inc.
  Ximian, Inc.

Most contributors on this list agreed to relicensing according to RELICENSE.md.

The following copyright holders did not answer the request for agreeing
to relicensing:

- <j(at)bootlab.org>: no contributions were made that are related to
  keyfile implementation. The script just gives a false positive.

- <pmarti(at)warp.es>: the contribution is a fix of a spelling error
  (commit 6029288ffb).

- <tgraf(at)redhat.com>: the contribution to keyfile code are small
  (I only identified commit 5b7503e95e).
  Also, Thomas worked for Red Hat at the time.

After research, I think it's fair to conclude that everybody who holds
non-trivial copyright on the keyfile code agreed to the relicensing.

libnm-core/nm-keyfile/nm-keyfile-internal.h

@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: GPL-2.0+ */
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /*
  * Copyright (C) 2008 Novell, Inc.
  * Copyright (C) 2015 Red Hat, Inc.

libnm-core/nm-keyfile/nm-keyfile-utils.c

@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: GPL-2.0+ */
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /*
  * Copyright (C) 2010 Red Hat, Inc.
  */

libnm-core/nm-keyfile/nm-keyfile-utils.h

@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: GPL-2.0+ */
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /*
  * Copyright (C) 2010 - 2015 Red Hat, Inc.
  */

libnm-core/nm-keyfile/nm-keyfile.c

@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: GPL-2.0+ */
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /*
  * Copyright (C) 2008 - 2009 Novell, Inc.
  * Copyright (C) 2008 - 2017 Red Hat, Inc.