Add API for 64-bit large object access. Now users can access up to

4TB large objects (standard 8KB BLCKSZ case).  For this purpose new
libpq API lo_lseek64, lo_tell64 and lo_truncate64 are added.  Also
corresponding new backend functions lo_lseek64, lo_tell64 and
lo_truncate64 are added. inv_api.c is changed to handle 64-bit
offsets.

Patch contributed by Nozomi Anzai (backend side) and Yugo Nagata
(frontend side, docs, regression tests and example program). Reviewed
by Kohei Kaigai. Committed by Tatsuo Ishii with minor editings.