ivgen: Allow reusing the same message ID twice in sequential IV gen

ivgen: Allow reusing the same message ID twice in sequential IV gen

We use the message ID and fragment number as IV generator. As IKEv2 uses
distinct message ID counters for actively and passively initiated exchanges,
each IV would be used twice. As we explicitly reject such message IDs since
d0ed1079, original-responder initiated exchanges fail with counter mode ciphers.

This commit separates IV space in two halves for sequential IVs, and
automatically assigns once reused sequence numbers to the second half.

Fixes #980.