Move Linux client & common packages into a public repo.

wgengine/rusage_windows.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.
+
+package wgengine
+
+func rusageMaxRSS() float64 {
+	// TODO(apenwarr): Substitute Windows equivalent of Getrusage() here.
+	return 0
+}