Prefer other types to int in names.c

Prefer other types to int in names.c

* src/names.c (uname_to_uid, gname_to_gid, handle_option)
(make_file_name): Prefer bool for boolean.
(struct name_elt, read_name_from_file): Prefer char for char.
(handle_option): Invert sense of return value, for clarity.
All uses changed.
(merge_sort_sll, merge_sort, collect_and_sort_names):
Don’t assume list length fits in int.  Use intptr_t not idx_t,
since the bound is the size of all memory rather than one array.