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_other.go

@@ -0,0 +1,10 @@
+// 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.
+
+// +build !darwin,!windows
+
+package dns
+
+func networkIsDown(err error) bool        { return false }
+func networkIsUnreachable(err error) bool { return false }