Imporved testing turned these up:
1) Headless+Selectorless, on a single-stack cluster, policy=PreferDual
Prior to this commit, the result was a single IPFamiliy (because we
checked that the 2nd allocator was present). This changes that case to
populate both families (we don't care if the allocator exists), which is
the same as RequireDual.
2) ClusterIP, user specifies 2 families but no IPs
Prior to this commit, the policy was inferred to be SingleStack. This
changes that case to correctly default to RequireDual when 2 families
are present but no IPs.
1e39f6ccf9