* Add SPDX License information
Adding license information according to the SPDX stanard.
Used the reuse tool.
Bug: https://github.com/scsitape/stenc/issues/95
* Add SPDX license information to remaining files
- converted AUTHORS to markdown
- added FSFUL license INSTALL.license as it is named in the header of INSTALL
- added SPDX license information for some files such as insignificant-files
and uncopyrightable files.
used GPL-2-or-later where possible, because this was the former
statement of the project and in order to minimize the number of licenses
used.
See also https://reuse.software/faq/#uncopyrightable
and https://reuse.software/tutorial/#insignificant-files
