reserve NebulaHandshakeDetails fields for multiport

We are currently testing changes for multiport (related to #497) that
use fields 6 and 7 in the protobuf. Reserved these fields so that when
we eventually open the PR we are backwards compatible with any future
changes.

Author: wadey

nebula.pb.go

@@ -61,5 +61,7 @@ message NebulaHandshakeDetails {
   uint32 ResponderIndex = 3;
   uint64 Cookie = 4;
   uint64 Time = 5;
+  // reserved for WIP multiport
+  reserved 6, 7;
 }