[$] Automatic buffer selection for io_uring

The io_uring subsystem has, in the lastyear, redefined how asynchronous I/O is done on Linux systems. As thissubsystem grows in both capability and users, though, it starts to run intolimitations in the types of operations that can be expressed. That isdriving a number of changes in how operations are programmed for io_uring.One example is the mechanisms considered for carrying a file descriptorbetween operations that was covered here in early March. Another has todo with how I/O buffers are chosen for operations.