Move wgengine/tsdns to net/dns.

Straight move+fixup, no other changes. In prep for merging with
wgengine/router/dns.

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

net/dns/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 dns
+
+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
+}