MFp4

MFp4
 - Each stp port is added sequentially so it was possible for our bridgeid to
   change every time because the new port has a lower MAC address.  Instead
   just find the lowest MAC address from all Ethernet adapters in the machine
   as the value only needs to be unique, this stops a lot of churn on the
   protocol.
 - Update the states after enabling or disabling a port.
 - Keep tabs if we have been stopped or started by our parent bridge.
 - The callout only needs to be drained before destroying the mutex, move it to
   bstp_detach.