OpenFile flag argument should be of type Flag not int

OpenFile flag argument should be of type Flag not int

https://github.com/parrogo/writefs/blob/8a31bcb0de3409bcbe9a14c037c56a93fd57ddc8/writefs.go#L113

```golang
}

// # TODO: OpenFile flag argument should be of type Flag not int

// OpenFile is the generalized open call; It opens the named file with
// specified flags (O_RDONLY etc.).
//
```

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