Update licensing info

Closes #238

Update all files to list SPDX license shortname.  Most files are
BSD 3 clause license.

The exceptions are:

serge@sl ~/src/shadow$ git grep SPDX-License | grep -v BSD-3-Clause
contrib/atudel:# SPDX-License-Identifier: BSD-4-Clause
lib/tcbfuncs.c: * SPDX-License-Identifier: 0BSD
libmisc/salt.c: * SPDX-License-Identifier: Unlicense
src/login_nopam.c: * SPDX-License-Identifier: Unlicense
src/nologin.c: * SPDX-License-Identifier: BSD-2-Clause
src/vipw.c: * SPDX-License-Identifier: GPL-2.0-or-later

Signed-off-by: Serge Hallyn <serge@hallyn.com>

src/vipw.c

@@ -1,27 +1,14 @@
 /*
   vipw, vigr  edit the password or group file
   with -s will edit shadow or gshadow file
-
-  Copyright (c) 1997       , Guy Maor <maor@ece.utexas.edu>
-  Copyright (c) 1999 - 2000, Marek Michałkiewicz
-  Copyright (c) 2002 - 2006, Tomasz Kłoczko
-  Copyright (c) 2007 - 2013, Nicolas François
-  All rights reserved.
-
-  This program 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 2 of the License, or
-  (at your option) any later version.
-
-  This program 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 this program; if not, write to the Free Software
-  Foundation, Inc., 51 Franklin Street, Fifth Floor,
-  Boston, MA 02110-1301, USA.  */
+ *
+ * SPDX-FileCopyrightText: 1997       , Guy Maor <maor@ece.utexas.edu>
+ * SPDX-FileCopyrightText: 1999 - 2000, Marek Michałkiewicz
+ * SPDX-FileCopyrightText: 2002 - 2006, Tomasz Kłoczko
+ * SPDX-FileCopyrightText: 2007 - 2013, Nicolas François
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
 
 #include <config.h>