Fix split mode for all protocols but DoH

In split mode, the code must check for ipv6 availability to return the
correct network stack. Otherwise, we may end up using "tcp6-tls" even
though the upstream IP is an ipv4.

dot.go

@@ -33,6 +33,7 @@ func (r *dotResolver) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, erro
 	endpoint := r.uc.Endpoint
 	if r.uc.BootstrapIP != "" {
 		dnsClient.TLSConfig.ServerName = r.uc.Domain
+		dnsClient.Net = "tcp-tls"
 		_, port, _ := net.SplitHostPort(endpoint)
 		endpoint = net.JoinHostPort(r.uc.BootstrapIP, port)
 	}