[$] Descriptorless files for io_uring

The lowly file descriptor is one of the fundamental objects in Linuxsystems. A file descriptor, which is a simple integer value, can refer to anopen file - or to a network connection, a running process, a loaded BPFprogram, or a namespace. Over the years, the use of file descriptors to refer to transient objectshas grown to the point that it can be difficult to justify an API thatuses anything else. Interestingly, though, the io_uring subsystem looks as if it is movingtoward its own number space separate from file descriptors.