Add new functions to extract {client,server}_random, master_key

Tor uses these values to implement a low-rent clone of RFC 5705 (which,
in our defense, we came up with before RFC 5705 existed).  But now that
ssl_st is opaque, we need another way to get at them.

Includes documentation, with suitable warnings about not actually
using these functions.

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