version: add IsMobile func

And use it control/controlclient.

version/prop.go

@@ -0,0 +1,15 @@
+// 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 version
+
+import "runtime"
+
+// IsMobile reports whether this is a mobile client build.
+func IsMobile() bool {
+	// Good enough heuristic for now, at least until Apple makes
+	// ARM laptops...
+	return runtime.GOOS == "android" ||
+		(runtime.GOOS == "darwin" && (runtime.GOARCH == "arm" || runtime.GOARCH == "arm64"))
+}