Fix committed. Thanks for the patches!

As Jesús and Terry have said, this won't be backported to 3.3/2.7, since
it is a new feature.


[oylenshpeegul]
| It's weird how different these three patches are! We're
| essentially doing the same thing: "please allow the x option to pass
| through to builtins.open." Why don't these three modules look more alike?

Mostly because they were written at different times, by different people,
with different things to be backward-compatible with. Ideally they would
share the bulk of their code, but it's tricky to do that without changing
behavior in some corner cases.