* tests/test_lobject.py (LargeObjectTests): add more tests,
        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.