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.

doc/stp.md

@@ -154,9 +154,10 @@ BPDUs toward the root, so the change reaches the rest of the network. With
 `stp version stp` the root port sends TCN instead, until a Config BPDU with
 TCA comes back.
 
-A TC flag received on a root or designated port is passed on the same way to
-the other ports, not back to the port it came in on. A legacy TCN is
-acknowledged with TCA and then treated like a received TC. A port that turns
+A TC flag received on a forwarding root or designated port is passed on the
+same way to the other ports, not back to the port it came in on; a port that
+is still discarding or learning drops it. A legacy TCN is acknowledged with
+TCA and then treated like a received TC. A port that turns
 alternate, loses its link, or is blocked for a loop flushes its own addresses
 without raising a topology change, and an edge port never raises one.