Prefer idx_t to size_t in wordsplit

Prefer idx_t to size_t in wordsplit

* gnulib.modules: Add ialloc.
* lib/wordsplit.c: Include ialloc.h.
(PRINTMAX): New constant.
(printflen, printfdots): New functions.
(wordsplit_dump_nodes, expvar, wordsplit_process_list): Use them.
(_wsplt_subsplit, wordsplit_string_unquote_copy):
Don’t limit lengths to INT_MAX.
(wordsplit_run): Remove.  All callers changed to use wordsplit_len.
(wordsplit_perror): Don’t limit lengths to ULONG_MAX.
* lib/wordsplit.c (wordsplit_init, alloc_space, struct wordsplit_node)
(wsnode_len, wordsplit_add_segm, coalesce_segment, wsnode_quoteremoval)
(wordsplit_finish, wordsplit_append, node_split_prefix)
(find_closing_paren, wordsplit_find_env, wsplt_assign_var)
(expvar, node_expand, expcmd, wordsplit_trimws)
(wordsplit_tildexpand, isglob, wordsplit_pathexpand)
(skip_sed_expr, skip_delim_internal, skip_delim)
(skip_delim_real, scan_qstring, scan_word)
(wordsplit_c_quoted_length, wordsplit_process_list)
(wordsplit_len, wordsplit_free_words, wordsplit_get_words):
* lib/wordsplit.h (struct wordsplit):
Prefer idx_t to size_t for indexes.
* lib/wordsplit.h: Include idx.h.