sepgsql: fix relkind handling on foreign tables

The attached patch fixes up case handling in foreign tables.

Now it didn't assign security label on foreign table on its creation
time, and didn't check access rights on the dml hook.
This patch fixes these problems; It allows foreign tables default
labeling and access checks as db_table object class.

Thanks,
-- 
KaiGai Kohei <kaigai@kaigai.gr.jp>