Add support for TLS_FALLBACK_SCSV (RFC7507)
This implements the TLS_FALLBACK_SCSV thingy both client and server side.
I'm not completely sure about the client side support though, in particular the way
gnutls_handshake_enable_fallback_scsv is implemented, but I couldn't come up with
To test the server support run
gnutls-serv and then connect to it with:
$ openssl s_client -connect localhost:5556 -fallback_scsv -no_tls1_2
To test the client support run:
$ gnutls-cli --priority="NORMAL:-VERS-TLS1.2" --fallback-scsv example.com
Fixes issue #9 (closed).