Flag absence of extended master secret extension

[drwetter]

This PR fixes #2806 and implements a feature request. TLS >=1.2 MUST support support the extended_master_secret extension to address an attack resulting from TLS session parameters not being properly authenticated in a Triple Handshake scenario (https://ieeexplore.ieee.org/document/6956559).

Only if the extension is missing there will be a medium severity level finding. JSON output will be generated in any case.

Also in determine_tls_extensions() some documentation about TLS extensions to be send were added.

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• For the main program: My edits contain no tabs, indentation is five spaces and any line endings do not contain any blank chars
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