net/dns: make MagicDNS IPv6 registration opt-out now, not opt-in

This adds a new ControlKnob to make MagicDNS IPv6 registration
(telling systemd/etc) opt-out rather than opt-in.

Updates #15404

Change-Id: If008e1cb046b792c6aff7bb1d7c58638f7d650b1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/manager_tcp_test.go

@@ -15,6 +15,7 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	dns "golang.org/x/net/dns/dnsmessage"
+	"tailscale.com/control/controlknobs"
 	"tailscale.com/health"
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsdial"
@@ -93,7 +94,8 @@ func TestDNSOverTCP(t *testing.T) {
 	bus := eventbustest.NewBus(t)
 	dialer := tsdial.NewDialer(netmon.NewStatic())
 	dialer.SetBus(bus)
-	m := NewManager(t.Logf, &f, health.NewTracker(bus), dialer, nil, nil, "", bus)
+	cknobs := &controlknobs.Knobs{}
+	m := NewManager(t.Logf, &f, health.NewTracker(bus), dialer, nil, cknobs, "", bus)
 	m.resolver.TestOnlySetHook(f.SetResolver)
 	m.Set(Config{
 		Hosts: hosts(