Add licensing and copyright information for all files in this repository. This
either happens in the file itself as a comment header or in the file
`.reuse/dep5`.
This commit also adds a Github workflow to check pull requests and adapts
copyright.pl to the changes.
Closes#8869
The hashes can vary between architectures (e.g. Sparc differs from x86_64).
This is not a fatal problem but just reduces the coverage of these white-box
tests, as the assumptions about into which hash bucket each key falls are no
longer valid.
