after various m_adj()s have been done. Kludge around this with a cheesy
macro that knows where the drivers put the mac header in the first mbuf.
XXX There should be a better way to do this.
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.
