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.h

@@ -488,8 +488,6 @@ public:
     }
 };
 
-bool NewThread(void(*pfn)(void*), void* parg);
-
 #ifdef WIN32
 inline void SetThreadPriority(int nPriority)
 {
@@ -500,7 +498,7 @@ inline void SetThreadPriority(int nPriority)
 #define THREAD_PRIORITY_LOWEST          PRIO_MAX
 #define THREAD_PRIORITY_BELOW_NORMAL    2
 #define THREAD_PRIORITY_NORMAL          0
-#define THREAD_PRIORITY_ABOVE_NORMAL    0
+#define THREAD_PRIORITY_ABOVE_NORMAL    (-2)
 
 inline void SetThreadPriority(int nPriority)
 {
@@ -512,11 +510,6 @@ inline void SetThreadPriority(int nPriority)
     setpriority(PRIO_PROCESS, 0, nPriority);
 #endif
 }
-
-inline void ExitThread(size_t nExitCode)
-{
-    pthread_exit((void*)nExitCode);
-}
 #endif
 
 void RenameThread(const char* name);