Fix PSK identity hint handling.

Fix PSK identity hint handling.

For server use a PSK identity hint value in the CERT structure which
is inherited when SSL_new is called and which allows applications to
set hints on a per-SSL basis. The previous version of
SSL_use_psk_identity_hint tried (wrongly) to use the SSL_SESSION structure.

PR#4039

Reviewed-by: Matt Caswell <matt@openssl.org>