Deduplicate peer certificate chain processing code.

Deduplicate peer certificate chain processing code.

Rather than reimplement this in each TLS client and server, deduplicate it
into a single function. Furthermore, rather than dealing with the API
hazard that is SSL_get_peer_cert_chain() in this code, simply produce two
chains - one that has the leaf and one that does not.
SSL_get_peer_cert_chain() can then return the appropriate one.

This also moves the peer cert chain from the SSL_SESSION to the
SSL_HANDSHAKE, which makes more sense since it is not available on
resumption.

ok tb@