Test for revival to a host behind NAT.

This uses auto=keep on east, meaning once road has initiated, east
wants the tunnel to remain up. Road then sends a delete to emulate
an idle connection being torn down. Normally, east cannot initiate to the
host behind NAT to keep the connection up. But if it is quick, it can
re-use the existing NAT mapping to establish a new tunnel.

auto=keep is the same as auto=add except it adds POLICY_UP to the
connection, so a delete will trigger the revive code.

NOTE: there is no ipsec auto --keep conn command yet

road uses a non-standard leftikeport, so nic does not assign port
4500 of road to its own port 4500, as that way we cannot test
that we are picking the right ephemeral port on east to initiate
to road to. We pick 6666 which nic will also NAT to its 6666.
