fuse.EROFS not defined

[DamonBlais]

As per the documentation on any write type action, this error code should be returned when attempts are made against a filesystem intended to be read-only (either through use of a mount option or by design.)

I could not find an error code matching syscall.EROFS defined in this package.

It should be defined.

[jacobsa]

Please don't open one issue per missing constant -- that's a little bit excessive.