Add SSL_get_client_ciphers() to return ciphers from ClientHello

On the server side, if you want to know which ciphers the client
offered, you had to use session->ciphers.  But that field is no
longer visible, so we need a method to get at it.

Signed-off-by: Nick Mathewson <nickm@torproject.org>
Signed-off-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>