Add --order-smallest-first and --order-largest-first options

main.hpp

@@ -66,6 +66,10 @@ struct order_field {
 
 extern std::vector<order_field> order_by;
 
+// not legal UTF-8, so can't appear as a real attribute name
+#define ORDER_BY_SIZE "\200size"
+extern bool order_by_size;
+
 int mkstemp_cloexec(char *name);
 FILE *fopen_oflag(const char *name, const char *mode, int oflag);
 bool progress_time();