cp: close file descriptors after cow on linux
cp: close file descriptors after cow on linux

Instead of using into_raw_fd(), which transfers ownership and
requires us to close the file descriptor manually,
use as_raw_fd(), which does not transfer ownership to us but drops the
file descriptor when the original file is dropped (in our case at the
end of the function).

uutils/coreutils,uutils/coreutils,uutils/coreutils