Avoid failing s_server when client's psk_identity is unexpected

s_server has traditionally been very brittle in PSK mode.  If the
client offered any PSK identity other than "Client_identity" s_server
would simply abort.

This is breakage for breakage's sake, and unlike most other parts of
s_server, which tend to allow more flexible connections.

This change accomplishes two things:

 * when the client's psk_identity does *not* match the identity
   expected by the server, just warn, don't fail.

 * allow the server to expect instead a different psk_identity from
   the client besides "Client_identity"

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3605)