Add ability to discard `A` or `AAAA` responses
Add ability to discard `A` or `AAAA` responses

Verizon FiOS still doesn't universally support IPv6, so I don't want to
get `AAAA` records at home.

Set the `IPV` environment variable to `4` or `6` to only fetch `A` or
`AAAA` records from upstream, respectively.

benizi/localdns