Add an ioctl example

[colinmarc]

This matches libfuse's example:

https://github.com/libfuse/libfuse/blob/d30247c36dadd386b994cd47ad84351ae68cc94c/example/ioctl.c

Unfortunately, it doesn't work at present. After mounting at /tmp/foobar, you can test it by compiling libfuse's ioctl client:

git clone https://github.com/libfuse/libfuse
cd libfuse/example
gcc -Wall ioctl_client.c `pkg-config fuse3 --cflags --libs` -o ioctl_client
./ioctl_client /tmp/foobar/fioc

This example returns ENOTTY without ever hitting the ioctl handler:

openat(AT_FDCWD, "/tmp/foobar/fioc", O_RDWR) = 3
ioctl(3, _IOC(_IOC_READ, 0x45, 0, 0x8), 0x7fffaf4404b0) = -1 ENOTTY (Inappropriate ioctl for device)

Whereas the libfuse example works:

ioctl(3, _IOC(_IOC_READ, 0x45, 0, 0x8), 0x7ffdb6c5b970) = 0

I'm happy to do the work for a fix, but I haven't been able to figure out where the bug is yet :)

[colinmarc]

Ok, figured it out. I needed --features abi-7-11 and to generate the ioctl codes correctly.