cmd/tailscaled: flush DNS if FlushDNSOnSessionUnlock is true upon receiving a session change notification

In this PR, we move the syspolicy.FlushDNSOnSessionUnlock check from service startup
to when a session change notification is received. This ensures that the most recent policy
setting value is used if it has changed since the service started.

We also plan to handle session change notifications for unrelated reasons
and need to decouple notification subscriptions from DNS anyway.

Updates #12687
Updates tailscale/corp#18342

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

cmd/tailscaled/tailscaled_windows.go

@@ -160,10 +160,7 @@ func (service *ipnService) Execute(args []string, r <-chan svc.ChangeRequest, ch
 	changes <- svc.Status{State: svc.StartPending}
 	syslogf("Service start pending")
 
-	svcAccepts := svc.AcceptStop
-	if flushDNSOnSessionUnlock, _ := syspolicy.GetBoolean(syspolicy.FlushDNSOnSessionUnlock, false); flushDNSOnSessionUnlock {
-		svcAccepts |= svc.AcceptSessionChange
-	}
+	svcAccepts := svc.AcceptStop | svc.AcceptSessionChange
 
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
@@ -371,13 +368,15 @@ func handleSessionChange(chgRequest svc.ChangeRequest) {
 		return
 	}
 
-	log.Printf("Received WTS_SESSION_UNLOCK event, initiating DNS flush.")
-	go func() {
-		err := dns.Flush()
-		if err != nil {
-			log.Printf("Error flushing DNS on session unlock: %v", err)
-		}
-	}()
+	if flushDNSOnSessionUnlock, _ := syspolicy.GetBoolean(syspolicy.FlushDNSOnSessionUnlock, false); flushDNSOnSessionUnlock {
+		log.Printf("Received WTS_SESSION_UNLOCK event, initiating DNS flush.")
+		go func() {
+			err := dns.Flush()
+			if err != nil {
+				log.Printf("Error flushing DNS on session unlock: %v", err)
+			}
+		}()
+	}
 }
 
 var (