Check for locks before using a display.

Patch by reinier@heeres.eu

sugar-emulator

@@ -39,15 +39,18 @@ def _get_display_number():
     display_is_free = False
 
     while not display_is_free and retries > 0:
+        lockstr = "/tmp/.X%d-lock" % display_number
+        if not os.path.exists(lockstr):
             s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
             try:
                 s.connect(('127.0.0.1', 6000 + display_number))
                 s.close()
+            except:
+                display_is_free = True
+                break
 
         display_number += 1
         retries -= 1
-        except:
-            display_is_free = True
 
     if display_is_free:
         log.info(