This uses splice on Linux and mmap+write elsewhere to transfer a file to
a pipe. Note we cannot use sendfile because (at least on macOS) the
receiving fd must be a socket.
One benefit of using `poll` over `select` is that we can re-use the poll
structure list. But there's no guarantee that the underlying system will
reset the `revents` field back to 0. So let's explicitly do so.
This is a remnant from SerenityOS. Let's avoid confusion as to why we
negate errno when we call Error::from_syscall just to negate it again
when we store the error code.
