Implement `AsFd` for `&T` and `&mut T`.

Add implementations of `AsFd` for `&T` and `&mut T`, so that users can
write code like this:

```rust
pub fn fchown<F: AsFd>(fd: F, uid: Option<u32>, gid: Option<u32>) -> io::Result<()> {
```

with `fd: F` rather than `fd: &F`.

And similar for `AsHandle` and `AsSocket` on Windows.

Also, adjust the `fchown` example to pass the file by reference. The
code can work either way now, but passing by reference is more likely
to be what users will want to do.

This is an alternative to #93869, and is a simpler way to achieve the
same goals: users don't need to pass borrowed-`BorrowedFd` arguments,
and it prevents a pitfall in the case where users write `fd: F` instead
of `fd: &F`.

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