cmd/tailscaled: use --statedir as documented

Enables the behavior described in the statepath flag, where if only
statedir is passed, then state is statedir/tailscaled.state.

Signed-off-by: James Tucker <james@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -137,7 +137,7 @@ func main() {
 	flag.StringVar(&args.httpProxyAddr, "outbound-http-proxy-listen", "", `optional [ip]:port to run an outbound HTTP proxy (e.g. "localhost:8080")`)
 	flag.StringVar(&args.tunname, "tun", defaultTunName(), `tunnel interface name; use "userspace-networking" (beta) to not use TUN`)
 	flag.Var(flagtype.PortValue(&args.port, 0), "port", "UDP port to listen on for WireGuard and peer-to-peer traffic; 0 means automatically select")
-	flag.StringVar(&args.statepath, "state", paths.DefaultTailscaledStateFile(), "absolute path of state file; use 'kube:<secret-name>' to use Kubernetes secrets or 'arn:aws:ssm:...' to store in AWS SSM; use 'mem:' to not store state and register as an emphemeral node. If empty and --statedir is provided, the default is <statedir>/tailscaled.state")
+	flag.StringVar(&args.statepath, "state", "", "absolute path of state file; use 'kube:<secret-name>' to use Kubernetes secrets or 'arn:aws:ssm:...' to store in AWS SSM; use 'mem:' to not store state and register as an emphemeral node. If empty and --statedir is provided, the default is <statedir>/tailscaled.state. Default: "+paths.DefaultTailscaledStateFile())
 	flag.StringVar(&args.statedir, "statedir", "", "path to directory for storage of config state, TLS certs, temporary incoming Taildrop files, etc. If empty, it's derived from --state when possible.")
 	flag.StringVar(&args.socketpath, "socket", paths.DefaultTailscaledSocket(), "path of the service unix socket")
 	flag.StringVar(&args.birdSocketPath, "bird-socket", "", "path of the bird unix socket")
@@ -187,6 +187,12 @@ func main() {
 		log.Fatalf("--bird-socket is not supported on %s", runtime.GOOS)
 	}
 
+	// Only apply a default statepath when neither have been provided, so that a
+	// user may specify only --statedir if they wish.
+	if args.statepath == "" && args.statedir == "" {
+		args.statepath = paths.DefaultTailscaledStateFile()
+	}
+
 	err := run()
 
 	// Remove file sharing from Windows shell (noop in non-windows)