ls: prefer xpalloc to xnrealloc
* src/ls.c (cwd_n_alloc, cwd_n_used, clear_files)
(extract_dirs_from_files, initialize_ordering_vector)
(update_current_files_info, sort_files, print_current_files)
(print_many_per_line, print_horizontal, print_with_separator)
(init_column_info, calculate_columns):
Prefer idx_t to size_t for sizes related to xpalloc.
(main): Let the compiler fold constants.
(gobble_file, init_column_info): Use xpalloc, not xnrealloc.
(print_many_per_line): Fix very-unlikely integer overflow.