Add support for magic cipher suite value (MCSV). Make secure renegotiation

work in SSLv3: initial handshake has no extensions but includes MCSV, if
server indicates RI support then renegotiation handshakes include RI.

NB: current MCSV value is bogus for testing only, will be updated when we
have an official value.

Change mismatch alerts to handshake_failure as required by spec.

Also have some debugging fprintfs so we can clearly see what is going on
if OPENSSL_RI_DEBUG is set.