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
Input challenges and returned messages are now in binary.
Conversions from/to base64 are performed by callers (currently curl_sasl.c
and http_ntlm.c).
Closes#6654
