MINOR: debug: make "show threads" properly iterate over all threads

Previously it would re-dump all threads to the same trash if  the output
buffer was full, which it never was since the trash is of the same size.
Now it dumps one thread, copies it to the buffer and yields until it can
continue. Showing 256 threads works as expected.

src/debug.c

@@ -370,14 +370,18 @@ static int cli_io_handler_show_threads(struct appctx *appctx)
 	else
 		thr = 0;
 
+	do {
 		chunk_reset(&trash);
-	ha_thread_dump_all_to_trash();
+		ha_thread_dump(&trash, thr);
 
 		if (applet_putchk(appctx, &trash) == -1) {
 			/* failed, try again */
 			appctx->st1 = thr;
 			return 0;
 		}
+		thr++;
+	} while (thr < global.nbthread);
+
 	return 1;
 }