impl IntoRawFd for TcpStream closes the fd

This code gets takes the TcpStream by value, gets its fd, and immediately destroys the TcpStream, likely destroying the fd:

```rust
impl IntoRawFd for TcpStream {
fn into_raw_fd(self) -> RawFd {
self.as_raw_fd()
}
}
```

It looks like the other impls of IntoRawFd do `self.watcher.into_inner().into_raw_fd()` which seems correct comparatively.

Start by locating the IntoRawFd implementation for TcpStream and compare it with the other implementations that call self.watcher.into_inner().into_raw_fd(). Verify the ownership-transfer behavior and check the relevant existing tests; the issue is done when converting a TcpStream leaves the returned file descriptor valid and owned by the caller.

Written by the indexing model from the issue text.