Move bwfm(4) from ifq begin/commit/rollback semantics to the newer

Move bwfm(4) from ifq begin/commit/rollback semantics to the newer
ifq dequeue semantics.  This basically means we need to check for
available space before dequeuing a packet.  As soon as we dequeue
a packet we commit to it.  On the PCIe backend this check can not
be done easily since the flowring depends on the packet contents and
we cannot take a peek.  When there is no flowring we cache the mbuf
and send it out as soon as the flowring opened up.  Then the ifq can
be restarted and traffic can flow.  Typically we usually run out of
packet ids, which can be checked without consulting the packet.  The
flowring probably never becomes full as the bwfm(4) firmware takes
the packets off the ring without actually sending them out.

Discussed with dlg@