kernel - Add a transparent MAC bridging feature to if_bridge

kernel - Add a transparent MAC bridging feature to if_bridge

* Defaults to non-transparent (historical) operation, which is safer.
  Set link0 to use in transparent MAC mode.

* Transparent MAC mode will attempt to retain the MAC source in the
  link address header when retransmitting a packet on a different
  interface.

  Only IP/IPV6 packets will retain the MAC.  ARP and other ether types
  will get the outgoing interface's MAC address, which is usually
  desireable.

* Note that transparent MAC mode is a bit dangerous, which is why it
  isn't turned on by default.  If a packet with the originating MAC
  winds up being sent out the same interface it came in on with the
  MAC intact, any switches between the two boxes will suddenly think
  the originating machine is somewhere else and will get confused.

  The code tries to avoid this situation.

  Bridging loops can also cause this sort of behavior even with the spanning
  tree protocol.  link0 is not recommended if you have loops.

* Coded because I needed this for braindead at&t uverse routers which
  do MAC-based security and only allow one IP association for each MAC,
  and whos firewalls cannot be completely disabled, and which cannot deal
  with IPs on routed networks (it expects everything to be directly connected
  on a switched network. sigh).