net/dns: add Windows group policy notifications to the NRPT rule manager

As discussed in previous PRs, we can register for notifications when group
policies are updated and act accordingly.

This patch changes nrptRuleDatabase to receive notifications that group policy
has changed and automatically move our NRPT rules between the local and
group policy subkeys as needed.

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

net/dns/manager_windows.go

@@ -297,7 +297,11 @@ func (m windowsManager) SupportsSplitDNS() bool {
 }
 
 func (m windowsManager) Close() error {
-	return m.SetDNS(OSConfig{})
+	err := m.SetDNS(OSConfig{})
+	if m.nrptDB != nil {
+		m.nrptDB.Close()
+	}
+	return err
 }
 
 // disableDynamicUpdates sets the appropriate registry values to prevent the