XQuartz: Fixed the "laggy" startup under the new model. This was caused by xinit blocking expecting SIGUSR1 and our stub never sending it.

(cherry picked from commit ee92aced10d0743c4658e53b58b5d9f5a094a415)

hw/xquartz/darwinEvents.c

@@ -350,6 +350,10 @@ Bool DarwinEQInit(void) {
     
     QuartzModeEQInit();
 
+    /* Note that this *could* cause a potential async issue, since we're checking
+     * darwinEvents without holding the lock, but darwinEvents is only ever set
+     * here, so I don't bother.
+     */
     if (!darwinEvents) {
         darwinEvents = (xEvent *)xcalloc(sizeof(xEvent), GetMaximumEventsNum());