The lobject.truncate(len=0) method will be available if psycopg2 has
been built against libpq from 8.3 or later (which is when the lobject
truncating support has been introduced).
including behaviour on closed lobjects and stale lobjects.
* psycopg/lobject_type.c (psyco_lobj_close): don't mark the
connection closed here because it is done by
lobject_close_locked().
* psycopg/lobject_int.c (lobject_open): mark objects as not closed
if we successfully open them.
(lobject_close_locked): mark the lobject closed here.
(lobject_export): ensure we are in a transaction, since
lo_export() issues multiple queries.
* psycopg/lobject_type.c (lobject_setup): make lobjects start closed.
