[sepgsql 2/3] Add db_schema:search permission checks

This patch adds sepgsql support for permission checks equivalent
to the existing SCHEMA USE privilege.

This feature is constructed on new OAT_SCHEMA_SEARCH event
type being invoked around pg_namespace_aclcheck().
So, its expected behavior also follows the behavior of existing
permissions; unprivileged schema is ignored from the search path,
or raise an error if object name is fully qualified.

This patch needs src/backend/catalog/objectaccess.c is existing,
so please apply this patch on top of this feature.
https://commitfest.postgresql.org/action/patch_view?id=1003

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