replace the hand rolled lists of mbufs in hfsc_classq with an

replace the hand rolled lists of mbufs in hfsc_classq with an
mbuf_list.

hfsc lists are very clever because they manage a fifo with a single
pointer by abusing the m_next pointer of the tail mbuf to point to
the head. clever but hard to read.

mbuf_lists are slightly bigger because they explicitely track the
head mbuf, but i got us that space back by inlining hfsc_classq
into hfsc_class and removing the unnecessary classq field.

ok henning@