SSL.handshake: Some client cert related fixes.

The client_cert_distinguished_names now contains DER-encoded DNs,
and not the decoded DNs.
Crypto.ECC.Curve.ECDSA: Added key_size().
SSL.session: MD5 is typically not supported with ECDSA.
SSL.handshake: Fix client cert exchange in TLS 1.2.

TLS 1.2 has a signature_algorithms array embedded in the
client certificate request.

Removes the combination MD5 + ECDSA from the set of suggested
signature algorithms, as we don't support it in certificates.

Also refrains from suggesting any ECDSA signature algorithms
if we don't support ECDSA.