Raw DNS forwarding keeps using startup nameservers after host DNS changes

Split out from #754, as discussed there.

#755 fixes AAAA queries by moving them to the host resolver. Other record types, including SOA, PTR, and CAA, still use raw forwarding with a DNS server list read only when gvproxy starts.

If the host switches DNS servers and the old servers become unreachable, those queries continue going to the old servers and fail. Disconnecting from a VPN with private DNS servers is one way this can happen; changing networks can also trigger it.

Sketch of an implementation here (https://github.com/cfergeau/gvisor-tap-vsock/compare/09-fixes...hs-matt:fix/dns-refresh-nameservers) that rereads the configuration for each forwarded query. Leaving this here for illustration purposes only.