Add lo_truncate() to backend and libpq for large object truncation.

doc/src/sgml/lobj.sgml

src/backend/libpq/be-fsstubs.c

src/backend/storage/large_object/inv_api.c

src/include/catalog/pg_proc.h

src/include/libpq/be-fsstubs.h

src/include/storage/large_object.h

src/interfaces/libpq/exports.txt

src/interfaces/libpq/fe-lobj.c

src/interfaces/libpq/libpq-fe.h

src/interfaces/libpq/libpq-int.h

src/test/regress/input/largeobject.source

src/test/regress/output/largeobject.source