kernel - Major bridging functionality added (bonding)

kernel - Major bridging functionality added (bonding)

* Add channel bonding support to if_bridge.  This utilizes the link2 flag
  on the bridge interface.  Participating member interfaces must be
  programmed to the same MAC address.  Multiple bonding groups can be
  created.

  Note that both sides of participating member interfaces must implement
  the link2 feature.  If the other side does not it will likely be blocking
  (due to the spanning tree protocol) some of the member interfaces and
  you will get packet loss.

* Document all the link flags and new features.