cmd/tailscale: refactor shared utility methods

Refactor two shared functions used by the tailscale cli,
calcAdvertiseRoutes and licensesURL. These are used by the web client as
well as other tailscale subcommands. The web client is being moved out
of the cli package, so move these two functions to new locations.

Updates tailscale/corp#13775

Signed-off-by: Will Norris <will@tailscale.com>

licenses/licenses.go

@@ -0,0 +1,21 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package licenses provides utilities for working with open source licenses.
+package licenses
+
+import "runtime"
+
+// LicensesURL returns the absolute URL containing open source license information for the current platform.
+func LicensesURL() string {
+	switch runtime.GOOS {
+	case "android":
+		return "https://tailscale.com/licenses/android"
+	case "darwin", "ios":
+		return "https://tailscale.com/licenses/apple"
+	case "windows":
+		return "https://tailscale.com/licenses/windows"
+	default:
+		return "https://tailscale.com/licenses/tailscale"
+	}
+}