dd: remove unnecessary fcntl dependency for stdout

[kimjune01]

Summary

dd fails in restricted containers where fcntl returns ENOSYS because try_clone_to_owned() internally calls fcntl. This was introduced in PR #10235.

• Replaces OwnedFileDescriptorOrHandle::from(io::stdout()) with a ManuallyDrop<File> wrapper that borrows the fd without cloning
• Eliminates the unnecessary fcntl syscall
• Works on both Unix (from_raw_fd) and Windows (from_raw_handle)

Test plan

• All 85 unit tests pass
• Basic functionality verified (echo "test" | dd bs=1)
• CI passes

Fixes #12157

[oech3]

Can we do same things without unsafe at least on unix?

[xtqqczze]

This is potentially unsound, we must guarantee stdout is never dropped.

[xtqqczze]

Can we do same things without unsafe at least on unix?

I don't think so, but we can use a safe wrapper: #12210

[oech3]

rustix::io::{read,write} does not require unsafe. So it should be able to make safe wrapper at least for unix and wasi without any unsafe.

[oech3]

struct RawReader<'a>(rustix::fd::BorrowedFd<'a>);
impl io::Read for RawReader<'_> {
    fn read(&mut self, b: &mut [u8]) -> io::Result<usize> {
        rustix::io::read(self.0, b).map_err(Into::into)
    }
}
let stdin = RawReader(rustix::stdio::stdin());

[Edit] this spreads <'a> to anywhere. So RawReader(std::io::stdin()) should be used.