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
... and as a practical side-effect, make sure that the
Curl_easyopts_check() function is asserted in debug builds, which we
want to detect mismatches between the options list in easyoptions.c and
the options in curl.h
Found-by: Gisle Vanem
Bug: 08e8455ddd (commitcomment-45991815)Closes#6461