issue 4: how to signal support for MOBIKE

Jari Arkko writes:
> So I would suggest that we use the Vendor ID approach.

I would suggest NOTIFY approach. I.e we send notify saying
MOBIKE_SUPPORTED and thats it. Similar thatn what is done for the
NAT-T or the IPCOMP_SUPPORTED or USE_TRANSPORT_MODE etc.

In most cases we do not want to fail the negotiation even if the other
end does not support MOBIKE, we simply revert back to redoing the
authentication every time IP-addresses changes.

If our final protocol is going to use notifies to send the
IP-addresses, then we can use those notifies to signal the use of
MOBIKE instead of separate MOBIKE_SUPPORTED notify.
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