cmd/tailscaled: use "Tailscale" as default TUN device name on Windows

That's what's used in the Windows GUI version and seems special. If we don't use
that, Windows tries to rename it and fails.

cmd/tailscaled/tailscaled.go

@@ -41,6 +41,17 @@ import (
 // later, the global state key doesn't look like a username.
 const globalStateKey = "_daemon"
 
+// defaultTunName returns the default tun device name for the platform.
+func defaultTunName() string {
+	switch runtime.GOOS {
+	case "openbsd":
+		return "tun"
+	case "windows":
+		return "Tailscale"
+	}
+	return "tailscale0"
+}
+
 func main() {
 	// We aren't very performance sensitive, and the parts that are
 	// performance sensitive (wireguard) try hard not to do any memory
@@ -50,15 +61,10 @@ func main() {
 		debug.SetGCPercent(10)
 	}
 
-	defaultTunName := "tailscale0"
-	if runtime.GOOS == "openbsd" {
-		defaultTunName = "tun"
-	}
-
 	cleanup := getopt.BoolLong("cleanup", 0, "clean up system state and exit")
 	fake := getopt.BoolLong("fake", 0, "fake tunnel+routing instead of tuntap")
 	debug := getopt.StringLong("debug", 0, "", "Address of debug server")
-	tunname := getopt.StringLong("tun", 0, defaultTunName, "tunnel interface name")
+	tunname := getopt.StringLong("tun", 0, defaultTunName(), "tunnel interface name")
 	listenport := getopt.Uint16Long("port", 'p', magicsock.DefaultPort, "WireGuard port (0=autoselect)")
 	statepath := getopt.StringLong("state", 0, paths.DefaultTailscaledStateFile(), "Path of state file")
 	socketpath := getopt.StringLong("socket", 's', paths.DefaultTailscaledSocket(), "Path of the service unix socket")