[Borgbackup] Official Windows Support

[Thomas Waldmann]

Another issue is testing.

For the UNIX-like platforms, we usually have some CI (travis-ci for
Linux and OS X) that tests every PR before merging.

The windows branch has AppVeyor for CI, 53 test failures currently.

There are also vagrant VMs that can be used for local / more practical
testing.

For windows (currently: only with cygwin), there's a vagrant machine,
but it lacks quite some usability / automation.

So, quite some TODO...

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