New system for tracking data-limit-based rekeys.

I've removed the encrypted_len fields from PktIn and PktOut, which
were used to communicate from the BPP to ssh.c how much each packet
contributed to the amount of data encrypted with a given set of cipher
keys. It seems more sensible to have the BPP itself keep that counter
- especially since only one of the three BPPs even needs to count it
at all. So now there's a new DataTransferStats structure which the BPP
updates, and ssh.c only needs to check it for overflow and reset the
limits.

ssh2bpp-bare.c

@@ -79,8 +79,6 @@ static void ssh2_bare_bpp_handle_input(BinaryPacketProtocol *bpp)
         s->pktin->refcount = 1;
         s->data = snew_plus_get_aux(s->pktin);
 
-        s->pktin->encrypted_len = s->packetlen;
-
         s->pktin->sequence = s->incoming_sequence++;
 
         /*