Don't complain and fail about unknown TLSv1.3 PSK identities in s_server

An unknown PSK identity could be because its actually a session resumption
attempt. Sessions resumptions and external PSKs are indistinguishable so
the callbacks need to fail gracefully if they don't recognise the identity.

Fixes #7433

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7434)