Add comments warning against generalizing default_with_oids.

pg_dump has historically assumed that default_with_oids affects only plain
tables and not other relkinds.  Conceivably we could make it apply to some
newly invented relkind if we did so from the get-go, but changing the
behavior for existing object types will break existing dump scripts.
Add code comments warning about this interaction.

Also, make sure that default_with_oids doesn't cause parse_utilcmd.c to
think that CREATE FOREIGN TABLE will create an OID column.  I think this is
only a latent bug right now, since we don't allow UNIQUE/PKEY constraints
in CREATE FOREIGN TABLE, but it's better to be consistent and future-proof.

src/backend/commands/tablecmds.c

@@ -579,6 +579,14 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 	 */
 	descriptor = BuildDescForRelation(schema);
 
+	/*
+	 * Notice that we allow OIDs here only for plain tables, even though some
+	 * other relkinds can support them.  This is necessary because the
+	 * default_with_oids GUC must apply only to plain tables and not any other
+	 * relkind; doing otherwise would break existing pg_dump files.  We could
+	 * allow explicit "WITH OIDS" while not allowing default_with_oids to
+	 * affect other relkinds, but it would complicate interpretOidsOption().
+	 */
 	localHasOids = interpretOidsOption(stmt->options,
 									   (relkind == RELKIND_RELATION));
 	descriptor->tdhasoid = (localHasOids || parentOidCount > 0);

src/backend/parser/parse_utilcmd.c

@@ -222,7 +222,18 @@ transformCreateStmt(CreateStmt *stmt, const char *queryString)
 	cxt.blist = NIL;
 	cxt.alist = NIL;
 	cxt.pkey = NULL;
-	cxt.hasoids = interpretOidsOption(stmt->options, true);
+
+	/*
+	 * Notice that we allow OIDs here only for plain tables, even though
+	 * foreign tables also support them.  This is necessary because the
+	 * default_with_oids GUC must apply only to plain tables and not any other
+	 * relkind; doing otherwise would break existing pg_dump files.  We could
+	 * allow explicit "WITH OIDS" while not allowing default_with_oids to
+	 * affect other relkinds, but it would complicate interpretOidsOption(),
+	 * and right now there's no WITH OIDS option in CREATE FOREIGN TABLE
+	 * anyway.
+	 */
+	cxt.hasoids = interpretOidsOption(stmt->options, !cxt.isforeign);
 
 	Assert(!stmt->ofTypename || !stmt->inhRelations);	/* grammar enforces */