Part of #117276.

The current organisation of the kernel_copy mechanism used to specialise io::copy on Linux necessitated circular links between the io::copy module and the implementation in sys::pal::unix::kernel_copy, as well as presenting an exception to the tidy PAL rule that forbids OS-based #[cfg]s outside of sys and os.

This PR fixes this by moving kernel_copy to sys (as per #117276) and returning a CopyState from that function specifying whether io::copy should use its fallback. The kernel_copy function on other platforms just unconditionally returns CopyState::Fallback.