rstp: Add admin-port-state setter.
rstp: Fix reception of BPDUs conveying an unkown port role.
rstp: Fix compare_rstp_priority_vector().

      The bridge_port_id is never conveyed in Configuration Messages, but is
      used as a tie-breaker within a Bridge.
rstp: Fix global transitions.
      When the condition associated with a global transition is met, it
      supersedes all other conditions.
rstp: fix setters. Invoke procedures only if values change.
rstp: Fix topology_change_sm transition.
rstp: Fix updt_roles_tree().
      If the DesignatedBridgeID Bridge Address component is equal to that
      component of the Bridge’s own bridge priority vector skip to the next
      port.