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>

lib/tcbfuncs.c

@@ -1,21 +1,6 @@
 /*
- * Copyright (c) 2001 Rafal Wojtczuk, Solar Designer
- * All rights reserved.
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted.
- *
- * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
- * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
- * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
- * PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT
- * HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
- * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
- * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
- * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
- * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
- * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
- * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ * SPDX-FileCopyrightText: 2001 Rafal Wojtczuk, Solar Designer
+ * SPDX-License-Identifier: 0BSD
  */
 
 #define _GNU_SOURCE