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
Introducing the curl bug bounty program on hackerone. We now recommend
filing security issues directly in the hackerone ticket system which
only is readable to curl security team members.
Assisted-by: Daniel Gustafsson
Closes#3488
