Fix wait loop and void cast (#24)

* Simplify EINTR/ECANCEL error handling

1. Add semaphore uninterruptible wait function
2 .Replace semaphore wait loop with a single uninterruptible wait
3. Replace all sem_xxx to nxsem_xxx

* Unify the void cast usage

1. Remove void cast for function because many place ignore the returned value witout cast
2. Replace void cast for variable with UNUSED macro

tools/lowhex.c

@@ -148,7 +148,7 @@ int main(int argc, char **argv)
             }
         }
 
-      (void)fputs(g_line, outstream);
+      fputs(g_line, outstream);
     }
 
   ret = 0;