ls: port to wider off_t, uid_t, gid_t

ls: port to wider off_t, uid_t, gid_t

* src/ls.c (dired_pos): Now off_t, not size_t, since it counts
output file offsets.
(dired_dump_obstack): This obstack's file offsets are now
off_t, not size_t.
(format_user_or_group, format_user_or_group_width):
ID arg is now uintmax_t, not unsigned long, since uid_t and
gid_t values might exceed ULONG_MAX.
(format_user_or_group_width): Use snprintf with NULL instead of
sprintf with a discarded buffer.  This avoids a stack buffer,
and so should be safer.