Fix TLS 1.2 and 1.3 detection against Windows Server 2019

[jtesta]

This fixes the issue reported in #271.

It was found that Windows Server 2019 + IIS 10 rejects all TLSv1.2 connections if the TLS_EMPTY_RENEGOTIATION_INFO_SCSV cipher is not present in the ClientHello ciphersuites. The fix is to include this cipher in the protocol detection phase.

Secondly, Windows Server 2019 + IIS 10 will respond to TLSv1.3 handshakes with a TLSv1.2 response when v1.3 is disabled (!). The solution is to parse the TLS extensions in the response and look for v1.3 explicitly listed in the supported_versions extension.

[rbsec]

Once again, Microsoft showing that standards for something for other people to follow....

Fantastic work as always - trying to work out what MS are doing is always a nightmare, so thanks so much for taking this on.