RLS: add pg_rowsecurity catalog
RLS: Add rowsec_relid to ResultRelInfo

This hunk may still be needed in the new approach to filter out rows that've
changed to no longer be visible due to user action r action of triggers;
we'd need to filter the result relation.
Add ALTER TABLE commands for row security
RLS: pg_dump support for dumping rowsecurity catalogs
RLS: psql support for reporting row security constraints
RLS: Enforce row-security by transforming query plans
RLS: Enforce row-security on COPY
RLS: Regression tests
RLS: Supplimental docs
RLS: Update and tidy up documentation
RLS: Add FAILING tests for SECURITY DEFINER in RLS

See 52689207.4060605@2ndquadrant.com
RLS: Add FAILING tests to demonstrate problem with FK constraints and superuser exemption