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>

libmisc/salt.c

@@ -1,3 +1,10 @@
+/*
+ * SPDX-FileCopyrightText:  Marek Michalkiewicz <marekm@i17linuxb.ists.pwr.wroc.pl>
+ * SPDX-FileCopyrightText:  J.T. Conklin <jtc@netbsd.org>
+ *
+ * SPDX-License-Identifier: Unlicense
+ */
+
 /*
  * salt.c - generate a random salt string for crypt()
  *