Make project REUSE compatible

The change was performed as follows:
- it was checked that the COPYING.MIT license was present with the
  initial commit and such each contributor was awere of the license
- mail was around to all previous contributors to check this assumption
  (mail was not replied by Hannah and Sebastian); every reply was a
  confirmation of the assumed MIT licensing
- all copyright information were recovered from the respective Git
  commits
- reuse lint was to used to check the REUSE conformance

.gitignore

@@ -1 +1,3 @@
+# SPDX-FileCopyrightText: none
+# SPDX-License-Identifier: CC0-1.0
 *.kdev4