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
Verify the change brought in commit 8e11731653061. It makes sure that
returning a failure from the progress callback even very early results
in the correct return code.
