No actual change, but the cert_st member of struct ssl_session_st is now

No actual change, but the cert_st member of struct ssl_session_st is now
called sess_cert instead of just cert.  This is in preparation of further
changes: Probably often when s->session->sess_cert is used, we should
use s->cert instead; s->session->sess_cert should be a new structure
containing only the stuff that is for just one connection (e.g.
the peer's certificate, which the SSL client implementations currently
store in s->session->[sess_]cert, which is a very confusing thing to do).
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