Fix #2992: don't create a named event

Evidently I misread the docs for CreateEvent: if you pass a name to
CreateEvent, then it creates a single shared system-wide Event with
that name.  So all Haskell processes on the machine were sharing the
same Event object.  duh.

rts/win32/ThrIOManager.c

@@ -34,7 +34,7 @@ getIOManagerEvent (void)
         hRes = CreateEvent ( NULL, // no security attrs
                              TRUE, // manual reset
                              FALSE, // initial state,
-                             "IO Manager Event" );
+                             NULL ); // event name: NULL for private events
         if (hRes == NULL) {
             sysErrorBelch("getIOManagerEvent");
             stg_exit(EXIT_FAILURE);