systat - Add -pftop feature to display active PF state table traffic

* Add the -pftop (:pftop) feature.  This feature displays all active
  PF states.  PF (packet filter) must be active for this feature to be
  useful.

  Only those states which are actively passing traffic are displayed,
  which makes this a lot more useful than a raw 'pfctl -s state' dump.

* Display is sorted by highest aggregate receive+transmit bandwidth.
systat - cleanup netbw/pftop

* Cleanup pass, fix a few bugs.
systat - cleanup netbw/pftop (2)

* Cleanup pass, fix a few bugs.