... since b8a3c3dd3f6c can mimic the behaviour of get_interface_ipv6()
we can now call interfaces_primary_address6() instead of the less
educated find_interface_ipv6*() equivalents.
Probably quite some people with WAN aliases run into this issue where
renewal doesn't work because rc.newwanipv6 latches on to the VIP instead
of the primary address.