Add in a little hack to tag mbufs with bridge directionality.
This allows for users of uinet_if_bridge to know what the origin interface
set was - internal or external interfaces.

* interfaces passed to uinet_if_bridge already have directionality
  tagged.
* change uinet_if_bridge to store the bif in the physical interface
  if_bridge parameter, rather than the bridge softc.
* This allows the input path to check the directionality of the
  interface.
* Whilst here, don't forward packets to interfaces that are the
  same directionality of the interface.
* Tag the bridge packet with an mbuf flag indicating the directionality.
* Percolate that through to l2info
* .. and into libuinet_api.