expand: prefer xpalloc to x2nrealloc

expand: prefer xpalloc to x2nrealloc

* src/expand-common.c (max_column_width, n_tabs_allocated)
(first_free_tab, add_tab_stop, parse_tab_stops, validate_tab_stops)
(get_next_tab_column):
Use idx_t for sizes.  All uses changed.
(add_tab_stop): Use xpalloc instead of X2NREALLOC.
Use ckd_add to check for overflow, instead of doing it by hand.