kernel - Major bridging functionality added (bug fixes 5)

kernel - Major bridging functionality added (bug fixes 5)

* Consolidate the unicast target interface selection code into a single
  routine.

* bridge_start() now calls the unicast target interface selection routine.
  Prior to this packets originated on the machine containing the bridge
  were not selecting the proper target interface when bonding was operational,
  and would also not select a backup interface if the learned target
  interface went offline.

* Add some debug calls to mbuftrackid()