ipn, wgengine, magicsock, tsdns: be quieter and less aggressive when offline

If no interfaces are up, calm down and stop spamming so much. It was
noticed as especially bad on Windows, but probably was bad
everywhere. I just have the best network conditions testing on a
Windows VM.

Updates #604

wgengine/tsdns/neterr_windows.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2020 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 tsdns
+
+import (
+	"net"
+	"os"
+
+	"golang.org/x/sys/windows"
+)
+
+func networkIsDown(err error) bool {
+	if oe, ok := err.(*net.OpError); ok && oe.Op == "write" {
+		if se, ok := oe.Err.(*os.SyscallError); ok {
+			if se.Syscall == "wsasendto" && se.Err == windows.WSAENETUNREACH {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+func networkIsUnreachable(err error) bool {
+	// TODO(bradfitz,josharian): something here? what is the
+	// difference between down and unreachable? Add comments.
+	return false
+}