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
When CURL_SOCKET_BAD is returned in the callback, it should be treated
as an error (CURLE_COULDNT_CONNECT) if no other socket is subsequently
created when trying to connect to a server.
Bug: http://curl.haxx.se/mail/lib-2015-06/0047.html
