[$] ioctl() for io_uring

Of all the system calls in the Unix tradition, few are as maligned as ioctl().But ioctl() exists for a reason - for many reasons, in truth - andcannot be expected to go away anytime soon. It is thus unsurprising thatthere is interest in providing ioctl()-like functionality in theio_uring subsystem. A recent RFC patch setfrom Jens Axboe shows the form that this feature might take in theio_uring context.