net/dns: disable DNS registration for Tailscale interface on Windows

We already disable dynamic updates by setting DisableDynamicUpdate to 1 for the Tailscale interface.
However, this does not prevent non-dynamic DNS registration from happening when `ipconfig /registerdns`
runs and in similar scenarios. Notably, dns/windowsManager.SetDNS runs `ipconfig /registerdns`,
triggering DNS registration for all interfaces that do not explicitly disable it.

In this PR, we update dns/windowsManager.disableDynamicUpdates to also set RegistrationEnabled to 0.

Fixes #13411

Signed-off-by: Nick Khyl <nickk@tailscale.com>

net/dns/manager_windows.go

@@ -469,6 +469,9 @@ func (m *windowsManager) disableDynamicUpdates() error {
 		}
 		defer k.Close()
 
+		if err := k.SetDWordValue("RegistrationEnabled", 0); err != nil {
+			return err
+		}
 		if err := k.SetDWordValue("DisableDynamicUpdate", 1); err != nil {
 			return err
 		}