Fixes for windows clients

[ethanbb]

This contains 2 small but important changes for windows clients.

1. The connection needs to use \n as the newline character to work with Linux servers; otherwise basically every command fails and the kernel never starts. (The Windows-to-Linux case is the only one I have tested.)
2. The PopenSpawn object does not come with a sendintr method, but it does have kill which can send any signal. This adds sendintr in the adapter object to restore the functionality of killing the kernel when the jupyter-lab process is interrupted.