changes to thread code (directly use boost::thread)

- removes our NewThread() function an replaces remaining calls with
  boost::thread with our TraceThread template
- remove ExitThread() function
- fix THREAD_PRIORITY_ABOVE_NORMAL for non Windows OSes

src/util.cpp

@@ -1468,15 +1468,3 @@ void RenameThread(const char* name)
     (void)name;
 #endif
 }
-
-bool NewThread(void(*pfn)(void*), void* parg)
-{
-    try
-    {
-        boost::thread(pfn, parg); // thread detaches when out of scope
-    } catch(boost::thread_resource_error &e) {
-        printf("Error creating thread: %s\n", e.what());
-        return false;
-    }
-    return true;
-}