Don't allow system columns in check constraint (Bug fix: Minor inheritance/check bug: Inconsistent behavior)

https://commitfest.postgresql.org/action/patch_view?id=1236
http://archives.postgresql.org/message-id/CAA4eK1+7A1FEY3NhhUxTPuY++fYpGcVgR0-tHBj3=vTZZatwog@mail.gmail.com