Add SPDX identifiers to files

This is a request from FreeBSD.
diff --git a/functions.sh b/functions.sh
index 254e81c..a6cf1c0 100755
--- a/functions.sh
+++ b/functions.sh
@@ -1,5 +1,7 @@
 #! /bin/sh
 #
+# SPDX-License-Identifier: BSD-2-Clause
+#
 # Copyright (c) 2018-2020 Gavin D. Howard and contributors.
 #
 # All rights reserved.