ikev2: Only add NAT-D notifies to DPDs as initiator

ikev2: Only add NAT-D notifies to DPDs as initiator

If a responder is natted it will usually be a static NAT (unless it's a
mediated connection) in which case adding these notifies makes not much
sense (if the initiator's NAT mapping had changed the responder wouldn't
be able to reach it anyway).  It's also problematic as some clients refuse
to respond to DPDs if they contain such notifies.

Fixes #2126.