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
configure --enable-debug now enables -Wassign-enum with clang,
identifying several enum "abuses" also fixed.
Reported-by: Gisle Vanem
Bug: 879007f811 (commitcomment-42087553)Closes#5929
