Fix Windows exit tunnel

[krallin]

It looks like we need to apply more force when shutting down OpenSSH on Windows, at least when using the Windows fork (https://github.com/PowerShell/Win32-OpenSSH / https://github.com/PowerShell/openssh-portable).

Using SIGINT, it looks like the OpenSSH process doesn't do anything.

Unfortunately, it appears this causes OpenSSH to not cleanly disconnect from the server.

This is not ideal, but it's probably acceptable for now since we'll stop sending keep-alives and the connection should end up shutting down on the server-side reasonably quickly (about 2 minute later).

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This should fix part of the issue reported in https://github.com/aptible/aptible-cli/issues/212

cc @fancyremarker

[codecov-io]

Current coverage is 89.90% (diff: 100%)

Merging #214 into master will decrease coverage by 0.01%

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  Misses          136        136          
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