multiport: say when lanes are on and what was negotiated

Every multiport log line so far was a reason it turned itself off, so a node
running it looked identical to one that never tried. Add the other half:

- an Info at startup with the lane count and port range
- the negotiated lanes on both "Handshake message received" lines, which
  required allocating the lane set before the log rather than after; it still
  lands before CheckAndComplete/Complete, which was the ordering requirement
- multiport.lanes.up and multiport.lanes.tunnels gauges

The lane field is a slog.Attr so it vanishes on a node without multiport, and
reads zeros when we offered lanes and the peer had none - the case an operator
is actually looking for.

The gauges walk the hostmap instead of keeping a counter at promote/demote: a
tunnel torn down while its lanes are up never demotes them, so a counter would
drift upward forever. Only a multiport node pays for the walk.

main.go

@@ -325,6 +325,11 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 		handshakeConfig.laneCount = lanes
 		handshakeConfig.lanePortCount = uint16(routines)
 		handshakeConfig.laneBasePort = uint16(port)
+
+		// Every other multiport log line is a reason it turned itself off, so say
+		// plainly when it is on and with what. A peer only gets lanes if it also
+		// advertises a port range, so this is our half of the negotiation.
+		l.Info("multiport enabled", "lanes", lanes, "basePort", port, "ports", routines)
 	}
 
 	handshakeManager := NewHandshakeManager(l, hostMap, lightHouse, udpConns[0], handshakeConfig)