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.

src/include/nodes/parsenodes.h

@@ -1514,6 +1514,8 @@ typedef enum AlterTableType
 	AT_ReplicaIdentity,			/* REPLICA IDENTITY */
 	AT_EnableRowSecurity,		/* ENABLE ROW SECURITY */
 	AT_DisableRowSecurity,		/* DISABLE ROW SECURITY */
+	AT_ForceRowSecurity,		/* FORCE ROW SECURITY */
+	AT_NoForceRowSecurity,		/* NO FORCE ROW SECURITY */
 	AT_GenericOptions			/* OPTIONS (...) */
 } AlterTableType;