Fewer uses of size_t in list.c

Fewer uses of size_t in list.c

* src/list.c (recent_long_name_blocks, recent_long_link_blocks)
(read_header, from_header, gid_from_header, major_from_header)
(minor_from_header, mode_from_header, off_from_header)
(time_from_header, uid_from_header, uintmax_from_header)
(tartime): Prefer idx_t to size_t.