More frequent interrupt handling during du

The check is done when traversing directories in preorder i.e before the files
inside are enumerated. Postorder traversal is unwinding, so mostly we are done
with the calculation and finishing off the thread.

src/nnn.c

@@ -5486,8 +5486,11 @@ static void *du_thread(void *p_data)
 	FTSENT *node;
 
 	while ((node = fts_read(tree))) {
-		if (node->fts_info & FTS_D)
+		if (node->fts_info & FTS_D) {
+			if (g_state.interrupt)
+				break;
 			continue;
+		}
 
 		sb = node->fts_statp;