Use ipv6 queries in delv +ns

Use ipv6 queries in delv +ns

`delv +ns` invokes the same code to perform name resolution as `named`,
but it neglected to set up an IPv6 dispatch object first. Consequently,
it was behaving more like `named -4`.

It now sets up dispatch objects for both address families, and performs
resolver queries to both v4 and v6 addresses, except when one of the
address families has been suppressed by using `delv -4` or `delv -6`.