title: Find a free X11 display number
body: <p>I have some unit tests that need an X11 display so I plan to start Xvfb before running them, but to start Xvfb I will need a free display number to connect it to. My best guess is to see what's free in <code>/tmp/.X11-unix</code> but I'm not sure how to handle the race if many tests try to start simultaneously.</p>
  
  <p>sshd must do this, does anyone know how?</p>