Right now it is painful to integrate openat() with the normal IO classes. You have to figure out the low-level flags yourself (i.e. replicate the logic and error handling from the FileIO constructor), then replicate the open() logic yourself (because you want to set the name attribute on the FileIO object before wrapping it).

Therefore it would be nice if the FileIO constructor and the open() function supported openat natively. I see two possibilities:
- allow a (dirfd, name) tuple for the first "file" argument
- allow an optional dirfd argument at the end of the arglist