Add license headers to code files
- Some of the license headers for black hat had gpl-3.0-or-later, which i changed to gpl-3.0-only to conform to the license in the spectral README
- I added license headers with gpl-3.0-only for all other code files by black hat
- Code by me is licensed as gpl-2.0-or-later
- I added (/relicensed) a few of @carlschwan 's files to gpl-2.0-or-later, as some of them already had those and to make the licenses more consistent. (@carlschwan please confirm you're ok with this)