tun: replace the DNS client in netstack with net.Resolver

Use the net.Resolver DNS client to send domain lookups to the server
specified as a CreateNetTUN parameter. The net package's DNS client
handles DNS request and response parsing when PreferGo is true.

Like the previous DNS client it replaces, the net.Resolver instances
also sends DNS queries over the WireGuard connection. Tested on and with
support from Fly.io.