MINOR: conn-stream: Add ISBACK conn-stream flag

CS_FL_ISBACK is a new flag, set on backend conn-streams. We must just be
careful to preserve this flag when the endpoint is detached from the
conn-stream.
diff --git a/dev/flags/flags.c b/dev/flags/flags.c
index ddfd879..50f48b1 100644
--- a/dev/flags/flags.c
+++ b/dev/flags/flags.c
@@ -219,6 +219,7 @@
 		printf("0\n");
 		return;
 	}
+	SHOW_FLAG(f, CS_FL_ISBACK);
 
 	if (f) {
 		printf("EXTRA(0x%08x)", f);