csi: fix bad implementation of CBT (back tab)

* It takes a parameter, that indicates the number of tab stops to move
  through
* Use the tab stops defined in the tab stops list, not hard coded mod
  8 columns.

vt.c

@@ -875,6 +875,7 @@ action(struct terminal *term, enum action _action, uint8_t c)
                     break;
                 }
             }
+            assert(new_col >= term->cursor.point.col);
             term_cursor_right(term, new_col - term->cursor.point.col);
             break;
         }