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.

dev/flags/flags.c

@@ -219,6 +219,7 @@ void show_cs_flags(unsigned int f)
 		printf("0\n");
 		return;
 	}
+	SHOW_FLAG(f, CS_FL_ISBACK);
 
 	if (f) {
 		printf("EXTRA(0x%08x)", f);