SSL: Implemented support for the fallback SCSV.

This adds support for the protocol extension SCSV specified in
draft-ietf-tls-downgrade-scsv as of 2014-07-04.

This protects clients renegotiating failed connections with lower
protocol versions from MITM downgrade attacks, by informing the
server that the client actually supports a higher protocol version
than the one it is currently using.