logpolicy: set log target on windows based on a registry key (#1542)

Signed-off-by: Christine Dodrill <xe@tailscale.com>

util/winutil/winutil_notwindows.go

@@ -0,0 +1,16 @@
+// 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.
+
+// +build !windows
+
+package winutil
+
+const RegBase = ``
+
+// GetRegString looks up a registry path in our local machine path, or returns
+// the given default if it can't.
+//
+// This function will only work on GOOS=windows. Trying to run it on any other
+// OS will always return the default value.
+func GetRegString(name, defval string) string { return defval }