multiport: fix review findings in lane probing and recv_error

- Don't probe lanes on a tunnel idle in both directions, and don't count lane
  probes or their acks as tunnel traffic. A lane's own keepalive was keeping
  idle tunnels from ever being dropped by tunnels.drop_inactive.
- Accept a recv_error from an up lane's target. The peer answers lane traffic
  from the lane socket it arrived on, and the spoof guard only accepted the
  base remote, so recv_errors for lane traffic were ignored.
- Keep the probe generation across a lane reset, so an ack for a probe sent
  before a roam can't promote the lane on the new path.
- Bring handshake.proto in line with payload.go: field 3 is reserved and
  TxLanes is field 4.

outside.go

@@ -186,7 +186,9 @@ func (f *Interface) readOutsidePackets(via ViaSender, packet []byte, rxc *rxCont
 	if lane == 0 {
 		f.handleHostRoaming(hostinfo, via)
 	}
-	f.connectionManager.In(hostinfo)
+	if !isLaneProbe(h.Type, h.Subtype) {
+		f.connectionManager.In(hostinfo)
+	}
 
 	switch h.Type {
 	case header.Message:
@@ -552,9 +554,10 @@ func (f *Interface) maybeSendRecvError(endpoint netip.AddrPort, index uint32, q
 	}
 }
 
-// sendRecvError replies from the socket the offending packet arrived on (q).
-// A lane peer's spoof guard compares our source addr against the lane's
-// remote, so a reply from the base port would be discarded.
+// sendRecvError replies from the socket the offending packet arrived on (q), so
+// the reply to lane traffic rides the reverse of the lane's own 4-tuple: the
+// only one the peer's NAT and stateful firewalls are known to let through. The
+// peer's handleRecvError accepts it from that lane's target.
 func (f *Interface) sendRecvError(endpoint netip.AddrPort, index uint32, q int) {
 	f.messageMetrics.Tx(header.RecvError, 0, 1)
 
@@ -591,7 +594,7 @@ func (f *Interface) handleRecvError(addr netip.AddrPort, h *header.H) {
 	}
 
 	hr := hostinfo.GetRemote()
-	if hr.IsValid() && hr != addr {
+	if hr.IsValid() && hr != addr && !hostinfo.lanes.isLaneRemote(addr) {
 		f.l.Info("Someone spoofing recv_errors?",
 			"addr", addr,
 			"hostinfoRemote", hr,