net: sockets: tls: Make socket options work with multi-client server

For options that return the handshake status, make sure they work in
multi-client DTLS server case, by returning the status for the session
that completed handshake most recently. For TLS and DTLS client cases
that should make no difference, as there should only be one session per
context.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>