Implement MOBIKE (RFC 4555) support in iked(8), with us acting as

Implement MOBIKE (RFC 4555) support in iked(8), with us acting as
responder.  In practice this support means that clients like iPhones
can roam in different networks (LTE, WiFi) and change their external
addresses without having to re-do the whole handshake.  It allows the
client to choose how and when to change the external tunnel endpoint
addresses on demand, depending on which network is better or even is
connected at all.

ok sthen@
tweaks from jmc@
tested by a handful