vlib rename vlib_frame_args(...) to vlib_frame_scalar_args(..)

Typically we have scalar_size == 0, so it doesn't matter
but vlib_frame_args was providing pointer to scalar frame
data, not vector data. To avoid future confusion function
is renamed to vlib_frame_scalar_args(...)

Change-Id: I48b75523b46d487feea24f3f3cb10c528dde516f
Signed-off-by: Damjan Marion <damarion@cisco.com>

docs/gettingstarted/developers/vnet.md

@@ -52,7 +52,7 @@ units to convert buffer indices to buffer pointers:
      u16 nexts[VLIB_FRAME_SIZE], *next;
 
      n_left_from = frame->n_vectors;
-     from = vlib_frame_args (frame);
+     from = vlib_frame_vector_args (frame);
 
      /* 
       * Convert up to VLIB_FRAME_SIZE indices in "from" to