I've been thinking about this a bit more. What do you think about
putting the following restrictions?
1. FUSE server can only support well-formed ioctls. At the kernel side,
the interfaces remains the same for both FUSE and CUSE but libfuse only
exports well-formed ioctl API.
2. ioctl can only be used by FUSE server running as root (would this be
necessary? I'm not sure. To me it seems all the necessary protections
are already there).