Revocation gnutls client

[Werxis]

Dependencies required to run the program:

• gnutls (3.7.8 used)
• curl (7.85.0 used)

Side notes:

• the program uses its own verification callback, which runs during the TLS handshake (this overrides the default callback) Docs
• it is, therefore, necessary to manually verify the certificates, check the hostname, and only then check the revocation statuses of the certificates (done) Docs
• the revocation check takes place for all certificates in the chain (except for the root certificate and whenever possible - for example, if OCSP-stapling does not send a stapled OCSP Response, then no verification could be done)

Actual problems:

• random nonce manually added to the OCSP Request, but not present in the OCSP Response (responder does not include the nonce extension)
• OCSP stapling sending only stapled response for leaf server's certificate (currently don't know if it is even possible to configure)

[mukrop]

Thanks for the polishing in the refactoring. In general, I like this code as well and the same comments apply as are those written in #141.