net/{neterror,dns/resolver}: move PacketWasTruncated to neterror from DNS code

And delete the unused code in net/dns/resolver/neterr_*.go.

Change-Id: Ibe62c486bacce2733eb9968c96a98cbbdb2758bd
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/neterror/neterror_windows.go

@@ -0,0 +1,17 @@
+// 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 neterror
+
+import (
+	"errors"
+
+	"golang.org/x/sys/windows"
+)
+
+func init() {
+	packetWasTruncated = func(err error) bool {
+		return errors.Is(err, windows.WSAEMSGSIZE)
+	}
+}