Allow IPsec interface to have SAs reset

Make it easier to integrate with external IKE daemon.
IPsec interfaces can have one or both SAs replaced after
creation. This allows for the possibility of setting a
new child SA on an interface when rekeying occurs. It also
allows for the possibility of creating an interface ahead
of time and updating the SA when parameters that are
negotiated during IKE exchange become known.

Change-Id: I0a31afdcc2bdff7098a924a51abbc58bdab2bd08
Signed-off-by: Matthew Smith <mgsmith@netgate.com>