Fewer uses of size_t in create.c

Fewer uses of size_t in create.c

* src/create.c (struct exclusion_tag, to_octal, tar_copy_str)
(tar_name_copy_str, to_base256, to_chars_subst, to_chars)
(gid_to_chars, major_to_chars, minor_to_chars, mode_to_chars)
(off_to_chars, time_to_chars, uid_to_chars, uintmax_to_chars)
(string_to_chars, start_private_header, write_gnu_long_link)
(split_long_name, write_ustar_long_name, simple_finish_header)
(dump_dir0, ensure_slash, create_archive):
Prefer idx_t to size_t.