Umbrella: Windows support

[aawsome]

This is the tracking issue to collect the needed steps in order to support windows as OS:

• [x] make rustic compile on windows rustic-rs/rustic#11
• [ ] treat non-unicode filenames and quoting of filenames (+link targets?) correctly
• [ ] treat file permissions correctly
• [ ] treat user/group correctly
• [ ] treat special files (device,...) correctly
• [x] handle paths (like C:...) in snapshots rustic-rs/rustic#510 and rustic-rs/rustic#542
• [x] handle repo path containing a drive (like -r C:\path\to\repo) rustic-rs/rustic#495
• [ ] handle links correctly
• [ ] document windows behavior
• [x] Correctly use a parent snapshot rustic-rs/rustic#542
• [ ] Add VSS support (how others do it)
• [ ] restore issues (e.g. rustic-rs/rustic#635)