Provide a way to determine our maximum legacy version.

Provide a way to determine our maximum legacy version.

With the introduction of TLSv1.3, we need the ability to determine our
maximum legacy version and to track our peer's maximum legacy version.
This is needed for both the TLS record layer when using TLSv1.3, plus
it is needed for RSA key exhange in TLS prior to TLSv1.3, where the
maximum legacy version is incorporated in the pre-master secret to
avoid downgrade attacks.

This unbreaks RSA KEX for the TLS client when the non-version specific
method is used with TLSv1.0 or TLSv1.1 (clearly no one does this).

ok tb@