[MINOR] Add down termination condition

If a connection is closed by because the backend became unavailable
then log 'D' as the termination condition.

Signed-off-by: Simon Horman <horms@verge.net.au>
diff --git a/src/checks.c b/src/checks.c
index 6b34a37..5bcf47c 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -371,6 +371,8 @@
 				buffer_shutw_now(session->req);
 				buffer_shutr_now(session->rep);
 				session->task->nice = 1024;
+				if (!(session->flags & SN_ERR_MASK))
+					session->flags |= SN_ERR_DOWN;
 				task_wakeup(session->task, TASK_WOKEN_OTHER);
 		}
 	}
diff --git a/src/log.c b/src/log.c
index 58d884a..02b6b20 100644
--- a/src/log.c
+++ b/src/log.c
@@ -52,7 +52,7 @@
 	"Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
 };
 
-const char sess_term_cond[8]  = "-cCsSPRI";	/* normal, CliTo, CliErr, SrvTo, SrvErr, PxErr, Resource, Internal */
+const char sess_term_cond[9]  = "-cCsSPRID";	/* normal, CliTo, CliErr, SrvTo, SrvErr, PxErr, Resource, Internal, Down */
 const char sess_fin_state[8]  = "-RCHDLQT";	/* cliRequest, srvConnect, srvHeader, Data, Last, Queue, Tarpit */
 
 /*