ikev2: Trigger make-before-break reauthentication instead of reauth task
ikev2: Allow task to skip exchange by setting undefined exchange type
ikev2: Schedule a make-before-break completion task to delete old IKE_SA
mem-pool: Pass the remote IKE address, to re-acquire() an address during reauth

With make-before-break IKEv2 re-authentication, virtual IP addresses must be
assigned overlapping to the same peer. With the remote IKE address, the backend
can detect re-authentication attempts by comparing the remote host address and
port. This allows proper reassignment of the virtual IP if it is re-requested.

This change removes the mem-pool.reassign_online option, as it is obsolete now.
IPs get automatically reassigned if a peer re-requests the same address, and
additionally connects from the same address and port.
testing: Add a test scenario for make-before-break reauth without a virtual IP
testing: Add a test scenario for make-before-break reauth using a virtual IP
testing: Be a little more flexible in testing for established CHILD_SA modes

As we now print the reqid parameter in the CHILD_SA details, adapt the grep
to still match the CHILD_SA mode and protocol.
testing: Update description and test evaluation of host2host-transport-nat

As we now reuse the reqid for identical SAs, the behavior changes for
transport connections to multiple peers behind the same NAT. Instead of
rejecting the SA, we now have two valid SAs active. For the reverse path,
however, sun sends traffic always over the newer SA, resembling the behavior
before we introduced explicit SA conflicts for different reqids.