maint: use more modern fseeko, rather than fseek

maint: use more modern fseeko, rather than fseek

* src/tac.c (temp_stream): Use fseeko, not fseek, on principle:
use the more modern interface.  In general it is better to avoid
fseek due to its ABI-imposed 4GiB limit on the "offset", here its
use was fine because the offset was always 0.  Using fseeko also
has the advantage of not triggering a GNULIB_POSIXCHECK warning.
Reported by Eric Blake in
http://thread.gmane.org/gmane.comp.gnu.coreutils.general/2426/focus=2489