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.