Flagging retransmissions in connection history.

This adds a t/T letter for the first TCP payload retransmission from
originator or responder, respectively.

Addresses BIT-977.

scripts/base/protocols/conn/main.bro

@@ -87,6 +87,7 @@ export {
 		## f       packet with FIN bit set
 		## r       packet with RST bit set
 		## c       packet with a bad checksum
+                ## t       packet with retransmitted payload
 		## i       inconsistent packet (e.g. FIN+RST bits set)
 		## q       multi-flag packet (SYN+FIN or SYN+RST bits set)
 		## ======  ====================================================