Correctly handle incoming retransmissions of SYN packets.

If the SYNACK got lost, the peer that initiated a channel would retransmit
the SYN packet. However, the responder would ignore the retransmitted SYN
packet, causing the channel to stall and eventually time out.

diff --git a/src/utcp b/src/utcp
index ae9033c7c9211f6e9dfa3d6b3dfeebd6e1d9ba00..25675cca79330f968b3fb106793a5265615f4df7 160000 (submodule)
--- a/src/utcp
+++ b/src/utcp
@@ -1 +1 @@
-Subproject commit ae9033c7c9211f6e9dfa3d6b3dfeebd6e1d9ba00
+Subproject commit 25675cca79330f968b3fb106793a5265615f4df7