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 test 676 to verify that setting CURLOPT_COOKIEFILE to NULL again clears
the cookiejar from memory.
Reported-by: Stefan Karpinski
Fixes#6889Closes#6891