crazy multiport stuff

(cherry picked from commit 0488793a62)

handshake/handshake.proto

@@ -23,7 +23,19 @@ message NebulaHandshakeDetails {
   // hand-written parser silently skips it on read.
   uint64 Cookie = 4 [deprecated = true];
   uint64 Time = 5;
+  // Multiport lane negotiation. Absent on hosts without multiport enabled;
+  // vanilla nebula treats 6 and 7 as unknown fields and skips them.
+  LaneDetails InitiatorLanes = 6;
+  LaneDetails ResponderLanes = 7;
   uint32 CertVersion = 8;
-  // reserved for WIP multiport
-  reserved 6, 7;
+}
+
+// LaneDetails advertises a host's multiport lane capability. On a base
+// handshake LaneIndex is 0 and PortCount/BasePort describe the sender's
+// consecutively bound UDP ports. On a lane handshake the initiator sets
+// LaneIndex to its (nonzero) lane number.
+message LaneDetails {
+  uint32 PortCount = 1;
+  uint32 BasePort = 2;
+  uint32 LaneIndex = 3;
 }