According to RFC 5996, we MAY send an INFORMATIONAL message having an
AUTHENTICATION_FAILED. We don't do any retransmits, though, but just close
the IKE_SA after one message has been sent, avoiding the danger that an
unauthenticated IKE_SA stays alive.
If it is set on an auth config IDr will not be sent, and later the configured
identity will not only be checked against the returned IDr, but also
against other identities contained in the responder's certificate.
With uniqueids=no the daemon still deletes any existing IKE_SA with the
same peer if an INITIAL_CONTACT notify is received. With this new option
it also ignores these notifies.