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

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>