Idea: Add an option to set the mode to fs::copy

ais523:

splice only works if at least one of the specified file descriptors refers to a pipe

Yeah, you are right, I overlooked that, and it seems to still apply (as the other functions had their constraints relaxed over time).

It would be possible to run two splices in separate threads in parallel, or let kernel threads run them via io_uring, but that would be quite a lot of work for a relatively rare operation.