stp: use the front panel number as the port ID

The port ID in the BPDUs was the chip port plus one, so on most boards
it did not match the port label, neither on the Spanning Tree page nor
on the neighbour. Send the front panel number instead, and 101 and up
for the LAGs, the same numbers the page already uses for the Port
column. A returned BPDU of our own is mapped back to the port by that
number.

Scenario 66 gives one port a front panel number that differs from its
chip port.

doc/stp.md

@@ -142,6 +142,8 @@ Among ports offering the same root, the lowest root path cost wins, then the
 designated bridge, the designated port ID, and finally this switch's own port
 ID. A worse BPDU from a different bridge does not replace what a port holds;
 a worse one from the same designated bridge and port does.
+The number in a port ID is the one on the front panel, and 101 and up for
+the LAGs, so it matches the port labels on both ends of a link.
 
 ## Topology changes