Add Windows-specific code to enable building on Windows

[spectrumjade]

These changes are largely factoring out Unix-specific code and substituting code that will work with Windows, enabled by Go's build flag when compiling for Windows. The primary use case is to use tsh natively on Windows.

Also included is a mechanism to interact with ssh-agent when running on Windows, including using named pipes to interact with Microsoft's new native OpenSSH as well as Cygwin's pseudo Unix domain sockets when running Git for Windows.

There are some changes to the vendoring that will have to be incorporated before this will build properly. Specifically, moby/moby/pkg/term is broken for Windows at the moment, and also does not include the required function SetWinsize. This one may need to be forked and corrected. Additionally, Microsoft/go-winio will need to be vendored.

[gravitational-jenkins]

Can one of the admins verify this patch?

[gravitational-jenkins]

Can one of the admins verify this patch?

[klizhentas]

Here's what we will do: @russjones

• Let's include it in 2.7.0 - review and provide feedback
• Let's make sure commands that we know won't work tsh ssh are explicitly disabled on windows (as they don't support term resizes for example)
• Make sure windows build is clean and passing, there were some problems with moby package being broken on windows