net/tstun: rename wrap_windows.go to tun_windows.go.

The code has nothing to do with wrapping, it's windows-specific
driver initialization code.

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

net/tstun/tun_windows.go

@@ -0,0 +1,24 @@
+// 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 tstun
+
+import (
+	"github.com/tailscale/wireguard-go/tun"
+	"github.com/tailscale/wireguard-go/tun/wintun"
+	"golang.org/x/sys/windows"
+)
+
+func init() {
+	var err error
+	tun.WintunPool, err = wintun.MakePool("Tailscale")
+	if err != nil {
+		panic(err)
+	}
+	guid, err := windows.GUIDFromString("{37217669-42da-4657-a55b-0d995d328250}")
+	if err != nil {
+		panic(err)
+	}
+	tun.WintunStaticRequestedGUID = &guid
+}