Do not include a timestamp in the Client/ServerHello Random field.

Instead, send random bytes, unless SSL_SEND_{CLIENT,SERVER}RANDOM_MODE
is set.

This is a forward-port of commits:
  4af79303
  f4c93b46
  3da721da
  25832701

While the gmt_unix_time record was added in an ostensible attempt to
mitigate the dangers of a bad RNG, its presence leaks the host's view
of the current time in the clear.  This minor leak can help
fingerprint TLS instances across networks and protocols... and what's
worse, it's doubtful thet the gmt_unix_time record does any good at
all for its intended purpose, since:

    * It's quite possible to open two TLS connections in one second.

    * If the PRNG output is prone to repeat itself, ephemeral
      handshakes (and who knows what else besides) are broken.