SSL.Cipher: Improved TLS 1.2 compatibility.

The TLS 1.2 prf is now derived from the hash function used for
the MAC for the suite, as this seems to be the convention for
all the suites that have been defined after TLS 1.2.

This simplifies implementation of the prf selection behaviour
specified eg for the suites defined in RFC 6367 3.3:

  When used with TLS versions prior to 1.2 (TLS 1.0 and TLS 1.1),
  the PRF is calculated as specified in the appropriate version
  of the TLS specification.
SSL.Constants: Enabled suites using ECDHE_RSA key exchange.

This adds support for some of the suites from RFC 4492,
RFC 5289 and RFC 6367.