ike-natd: Create fake NAT-D payloads in a more static way

ike-natd: Create fake NAT-D payloads in a more static way

In some scenarios an IKE_SA might get restarted multiple times (e.g.
due to retransmits and delayed INVALID_KE_PAYLOAD notifies) so that
two IKE_SA_INIT messages might be sent that only differ in the
previously randomly generated NAT_DETECTION_SOURCE_IP payload.
This could cause an authentication failure on the responder if the two
peers don't use the same IKE_SA_INIT message in their InitiatorSignedOctets.

While the payload is generated in a reproducible way it will still change
when the daemon is restarted, which should make detecting the payloads
as fake a bit harder (compared to e.g. just using 0.0.0.0:0 as address).

Fixes #1131.