Add license headers on all files

[sylvestre]

No description provided.

[tertsdiepraam]

Do you think we can also standardize the headers that explicitly mention authors? In general, copyright is already included in Git and if we really wanted to mention all the contributors, we'd have to add a lot more authors 😄

[sylvestre]

Do you think we can also standardize the headers that explicitly mention authors?

yes, I would like to remove the author list too.
it doesn't provide value, it is outdated, we don't have rules to update it and as you said, it is clearly outdated

@tertsdiepraam @cakebaker ok if I remove them?

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail-2/inotify-dir-recreate