ALTER TABLE .. FORCE ROW LEVEL SECURITY

To allow users to force RLS to always be applied, even for table owners,
add ALTER TABLE .. FORCE ROW LEVEL SECURITY.

row_security=off overrides FORCE ROW LEVEL SECURITY, to ensure pg_dump
output is complete (by default).

Also add SECURITY_NOFORCE_RLS context to avoid data corruption when
ALTER TABLE .. FORCE ROW SECURITY is being used. The
SECURITY_NOFORCE_RLS security context is used only during referential
integrity checks and is only considered in check_enable_rls() after we
have already checked that the current user is the owner of the relation
(which should always be the case during referential integrity checks).

Back-patch to 9.5 where RLS was added.
