FUSE API changes

[Tommi Virtanen]

Heads up: just about all of the bazil.org/fuse/fs handlers changed their function call signature.

- Use contexts instead of an interrupt channel. Methods that looked like `Op(..., intr fs.Intr)` now look like `Op(ctx context.Context, ...)`.

- Remove fuse.Error, it has been useless for a while but removing it was a significant change, so bundle up with the above.

- Rename ReadDir to ReadDirAll, HandleReadDirer to HandleReadDirAller

    

    Setting the stage for a better ReadDir. The current implementation

    buffers the marshaled version of the whole directory in memory, and

    this can be costly both in RAM and in latency. Later, ReadDir (and

    maybe more ReadDirXxx style helpers) will be added.

    https://github.com/bazillion/fuse/issues/58

The commit messages contain shell snippets that can help you make the required changes.