d00f b86fd37eaa stp: pass a topology change on only from a forwarding port ...

stp_in() propagated the TC flag of any BPDU that did not arrive on an
alternate port, and acknowledged and propagated a TCN from any port, so a
port still in its listen period or held discarding in a dispute could flush
the addresses of every other port and move the topology change counter.
802.1D-2004 17.31 keeps the Topology Change machine in LEARNING until the
port forwards and drops rcvdTc and rcvdTcn there. Both paths now ask for
the forwarding state first; an alternate port is discarding, so the old
check is covered by the new one.

Scenario 53 plays a TC flag and a TCN into a port that is still discarding,
and then into the same port once it forwards. Scenario 51 used a TCN on a
port one tick short of its edge transition as the trigger for the clock it
tests; it now waits for the transition.

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..

Makefile

test/stp: build the simulator against the firmware headers

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stp_sim.c

stp: pass a topology change on only from a forwarding port

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