Provide pluggable queueing interface for pf

Provide pluggable queueing interface for pf

By hiding H-FSC behind pfq_ops structure similar to the ifq_ops,
we provide a possibility to plug alternative queueing interfaces
for use in pf.  This reduces amount of H-FSC specific code in the
pf ioctl handler

While here, change the the order of elements in hfsc_class_stats
to provide some compatibility between queue stat structures of
different traffic conditioners.

No objections from henning@, ok sthen@