New structure type SESS_CERT used instead of CERT inside SSL_SESSION.

New structure type SESS_CERT used instead of CERT inside SSL_SESSION.
While modifying the sources, I found some inconsistencies on the use of
s->cert vs. s->session->sess_cert; I don't know if those could
really have caused problems, but possibly this is a proper bug-fix
and not just a clean-up.