SSL: Support the truncated_hmac extension.

NB: Currently only implemented server-side.
SSL.handshake: Attempt to detect broken Safari 10.8.

The Safari browser versions 10.8.0..10.8.3 have broken support
for the ECDHE_ECDSA key exchange method, but advertise such
cipher suites anyway. Attempt to fingerprint the browser by
looking at its set of extensions, and filter the suites in
that case.

Adapted heuristics from OpenSSL 1.0.1f.
SSL.session: Make is_supported_suite() visible.

is_supported_suite() is used from SSL.handshake()->handle_handshake(),
so it needs to be visible.
SSL.Cipher: RSA server params are export mode only.

RFC 2246 7.4.3:
  It is not legal to send the server key exchange message for the
  following key exchange methods:

    RSA
    RSA_EXPORT (when the public key in the server certificate is
    less than or equal to 512 bits in length)
SSL.Cipher: Rotate export RSA keys.

The export RSA key is now regenerated after 5 uses.