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 API is soon two years old and deserves being shown as the primary
way to drive multi code as it makes it much easier to write code.
multi-poll: removed
multi-legacy: add to show how we did multi API use before
curl_multi_wait/poll.
Closes#7352