Added SPDX code to source files, LICENSE file

Make licensing clearer and (hopefully?) help GitHub detect the
    appropriate license.

LICENSE

@@ -0,0 +1,11 @@
+Copyright (c) 2023 Joshua Lee Ockert <torstenvl@gmail.com>
+
+THIS WORK IS PROVIDED "AS IS" WITH NO WARRANTY OF ANY KIND. THE IMPLIED
+WARRANTIES OF MERCHANTABILITY, FITNESS, NON-INFRINGEMENT, AND TITLE ARE
+EXPRESSLY DISCLAIMED. NO AUTHOR SHALL BE LIABLE UNDER ANY THEORY OF LAW
+FOR ANY DAMAGES OF ANY KIND RESULTING FROM THE USE OF THIS WORK.
+
+Permission to use, copy, modify, and/or distribute this work for any
+purpose is hereby granted, provided this notice appears in all copies.
+
+SPDX-License-Identifier: ISC

dirdedupe.sh

@@ -9,6 +9,8 @@
 #
 # Permission to use, copy, modify, and/or distribute this work for any
 # purpose is hereby granted, provided this notice appears in all copies.
+#
+# SPDX-License-Identifier: ISC
 
 
 ############################################################################

tmbless.sh

@@ -9,6 +9,8 @@
 #
 # Permission to use, copy, modify, and/or distribute this work for any
 # purpose is hereby granted, provided this notice appears in all copies.
+#
+# SPDX-License-Identifier: ISC
 
 
 function dispusage() {

tmimport.sh

@@ -9,6 +9,8 @@
 #
 # Permission to use, copy, modify, and/or distribute this work for any
 # purpose is hereby granted, provided this notice appears in all copies.
+#
+# SPDX-License-Identifier: ISC
 
 
 function dispusage() {