Problem: `chansend()` on Windows sends lines in reverse order.
Cause: Using \n instead of \r\n for newlines on Windows.
Solution: on Windows, use CRLF newline characters.
Fixes#18501
- Fix the problem that chanclose() does not work for channel created by
nvim_open_term().
- Fix the problem that the loopback channel is not released.
- Fix the error message when sending raw data to the loopback channel.