Bro Error when using IPv6 DNS server

For the moment, I've fixed this (in master) by searching for the first IPv4 address found in the resolver config.

Was going to try actually supporting IPv6, but the resolve.h API is unclear about whether it supports it. Clearly it was originally designed with only IPv4 in mind and I've googled and found references to extensions that allow it to do IPv6, except I don't understand whether they're fully implemented/available everywhere (at least on macOS, it looked like the extensions were declared, but maybe not defined for public use).