Provide option to embed libssh2

For those who need to build on Windows, provide a way to embed the libssh2 code into our own so we can have it without worrying about calling convention or building the external library with a name that doesn't conflict with some random dll someone built with a different convention.

This gives us a ton of warnings, but we should be able to add a bunch of `#pragma`s. I haven't tested it *that* much, but it does successfully fail to log into GitHub with a nonsensical auth combination.

This needs libssh2 from its master branch.