Prefer idx_t to size_t in tar.c

Prefer idx_t to size_t in tar.c

* src/tar.c (strip_name_components, archive_names)
(allocated_archive_names, tar_list_quoting_styles)
(expand_pax_option, parse_opt):
Prefer idx_t to size_t.
(decode_options): Use a static word rather than going
to to the bother of dynamically allocating an array.
(main): Do not preallocate array.  Do not call ‘free’
on a pointer that now might be to static storage.