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

mm/iob/iob_alloc_qentry.c

@@ -130,26 +130,8 @@ static FAR struct iob_qentry_s *iob_allocwait_qentry(void)
        * semaphore count will be incremented.
        */
 
-      ret = nxsem_wait(&g_qentry_sem);
-      if (ret < 0)
-        {
-          /* EINTR is not an error!  EINTR simply means that we were
-           * awakened by a signal and we should try again.
-           *
-           * REVISIT:  Many end-user interfaces are required to return
-           * with an error if EINTR is set.  Most uses of this function
-           * is in internal, non-user logic.  But are there cases where
-           * the error should be returned.
-           */
-
-          if (ret == -EINTR)
-            {
-              /* Force a success indication so that we will continue looping. */
-
-              ret = OK;
-            }
-        }
-      else
+      ret = nxsem_wait_uninterruptible(&g_qentry_sem);
+      if (ret >= 0)
         {
           /* When we wake up from wait successfully, an I/O buffer chain container was
            * freed and we hold a count for one IOB.  Unless somehting