Move DNS flush logic to net/dns.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/flush_windows.go

@@ -0,0 +1,19 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dns
+
+import (
+	"fmt"
+	"os/exec"
+)
+
+// Flush clears the local resolver cache.
+func Flush() error {
+	out, err := exec.Command("ipconfig", "/flushdns").CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("%v (output: %s)", err, out)
+	}
+	return nil
+}