directly, call the function pointer (*if_input)(ifp, m). The input routine
expects the packet header to be at the head of the packet, and will adjust
as necessary. Privatize the layer 2 input and output routines, allowing
*_ifattach() to set them up as appropriate.
return 0 instead of EINVAL. In this case, there will only be one media
type, and the upper-level if_media code will ensure that the user is setting
to that type.
In other words, don't require the front-end to provide the no-op.
thorpej