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
Add curl_url_strerror() to convert CURLUcode into readable string and
facilitate easier troubleshooting in programs using URL API.
Extend CURLUcode with CURLU_LAST for iteration in unit tests.
Update man pages with a mention of new function.
Update example code and tests with new functionality where it fits.
Closes#7605
