net/dns: retrample resolve.conf when another process has trampled it (#18069)

When using the resolve.conf file for setting DNS, it is possible that
some other services will trample the file and overwrite our set DNS
server. Experiments has shown this to be a racy error depending on how
quickly processes start.

Make an attempt to trample back the file a limited number of times if
the file is changed.

Updates #16635

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

net/dns/manager_windows_test.go

@@ -134,7 +134,7 @@ func TestManagerWindowsGPCopy(t *testing.T) {
 	}
 	defer delIfKey()
 
-	cfg, err := NewOSConfigurator(logf, nil, policyclient.NoPolicyClient{}, nil, fakeInterface.String())
+	cfg, err := NewOSConfigurator(logf, nil, nil, policyclient.NoPolicyClient{}, nil, fakeInterface.String())
 	if err != nil {
 		t.Fatalf("NewOSConfigurator: %v\n", err)
 	}
@@ -263,7 +263,7 @@ func runTest(t *testing.T, isLocal bool) {
 	}
 	defer delIfKey()
 
-	cfg, err := NewOSConfigurator(logf, nil, policyclient.NoPolicyClient{}, nil, fakeInterface.String())
+	cfg, err := NewOSConfigurator(logf, nil, nil, policyclient.NoPolicyClient{}, nil, fakeInterface.String())
 	if err != nil {
 		t.Fatalf("NewOSConfigurator: %v\n", err)
 	}