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

graphics/nxterm/nxterm_putc.c

@@ -183,5 +183,5 @@ void nxterm_showcursor(FAR struct nxterm_state_s *priv)
 
 void nxterm_hidecursor(FAR struct nxterm_state_s *priv)
 {
-  (void)nxterm_hidechar(priv, &priv->cursor);
+  nxterm_hidechar(priv, &priv->cursor);
 }

graphics/nxterm/nxterm_redraw.c

@@ -94,17 +94,6 @@ void nxterm_redraw(NXTERM handle, FAR const struct nxgl_rect_s *rect, bool more)
   do
     {
       ret = nxterm_semwait(priv);
-
-      /* Check for errors */
-
-      if (ret < 0)
-        {
-          /* The only expected error is if the wait failed because of it
-           * was interrupted by a signal.
-           */
-
-          DEBUGASSERT(ret == -EINTR || ret == -ECANCELED);
-        }
     }
   while (ret < 0);
 
@@ -125,5 +114,5 @@ void nxterm_redraw(NXTERM handle, FAR const struct nxgl_rect_s *rect, bool more)
       nxterm_fillchar(priv, rect, &priv->bm[i]);
     }
 
-  (void)nxterm_sempost(priv);
+  nxterm_sempost(priv);
 }

graphics/nxterm/nxterm_resize.c

@@ -88,11 +88,6 @@ int nxterm_resize(NXTERM handle, FAR const struct nxgl_size_s *size)
   ret = nxterm_semwait(priv);
   if (ret < 0)
     {
-      /* The only expected error is if the wait failed because of it was
-       * interrupted by a signal or if the thread was canceled.
-       */
-
-      DEBUGASSERT(ret == -EINTR || ret == -ECANCELED);
       return ret;
     }
 
@@ -103,6 +98,6 @@ int nxterm_resize(NXTERM handle, FAR const struct nxgl_size_s *size)
   priv->wndo.wsize.w = size->w;
   priv->wndo.wsize.h = size->h;
 
-  (void)nxterm_sempost(priv);
+  nxterm_sempost(priv);
   return true;
 }

graphics/nxterm/nxterm_unregister.c

@@ -90,7 +90,7 @@ void nxterm_unregister(FAR struct nxterm_state_s *priv)
   /* Unregister the driver */
 
   snprintf(devname, NX_DEVNAME_SIZE, NX_DEVNAME_FORMAT, priv->minor);
-  (void)unregister_driver(devname);
+  unregister_driver(devname);
 
   /* Free the private data structure */