various: general adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

No functional change intended.

kerberos5/lib/libgssapi_krb5/gss_krb5.c

@@ -1,4 +1,6 @@
 /*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
  * Copyright (c) 2005 Doug Rabson
  * All rights reserved.
  *

kerberos5/lib/libgssapi_krb5/pname_to_uid.c

@@ -1,4 +1,6 @@
 /*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
  * Copyright (c) 2008 Isilon Inc http://www.isilon.com/
  * Authors: Doug Rabson <dfr@rabson.org>
  * Developed with Red Inc: Alfred Perlstein <alfred@freebsd.org>

kerberos5/lib/libgssapi_krb5/prefix.c

@@ -1,4 +1,6 @@
 /*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
  * Copyright (c) 2008 Doug Rabson
  * All rights reserved.
  *

kerberos5/lib/libgssapi_ntlm/prefix.c

@@ -1,4 +1,6 @@
 /*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
  * Copyright (c) 2008 Doug Rabson
  * All rights reserved.
  *

kerberos5/lib/libgssapi_spnego/freebsd_compat.c

@@ -1,4 +1,6 @@
 /*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
  * Copyright (c) 2008 Doug Rabson
  * All rights reserved.
  *