[$] Descriptorless files for io_uring

The lowly file descriptor is one of the fundamental objects in Linux
systems. A file descriptor, which is a simple integer value, can refer to an
open file — or to a network connection, a running process, a loaded BPF
program, or a namespace.
Over the years, the use of file descriptors to refer to transient objects
has grown to the point that it can be difficult to justify an API that
uses anything else. Interestingly, though, the io_uring subsystem looks as if it is moving
toward its own number space separate from file descriptors.