Track the total size of every PacketQueue.

The queue-node structure shared between PktIn and PktOut now has a
'formal_size' field, which is initialised appropriately by the various
packet constructors. And the PacketQueue structure has a 'total_size'
field which tracks the sum of the formal sizes of all the packets on
the queue, and is automatically updated by the push, pop and
concatenate functions.

No functional change, and nothing uses the new fields yet: this is
infrastructure that will be used in the next commit.

ssh2bpp.c

@@ -589,6 +589,7 @@ static void ssh2_bpp_handle_input(BinaryPacketProtocol *bpp)
             continue;
         }
 
+        s->pktin->qnode.formal_size = get_avail(s->pktin);
         pq_push(&s->bpp.in_pq, s->pktin);
 
         {