use VSS/Volume Shadow Service under Windows to handle open files/directories

[daniel-l]

Is your feature request related to a problem? Please describe. Currently, when backing up Windows-User folders backrests throws a lot of hints at you when it tries to backup files which are in use by open programs. It is quite cumbersome to add exceptions for all these files. If backrest used VSS/Volume Copy Service there would by no need to monitor the backups for not handled open files (and add exceptions for them).

Another project using restic für backups on windows machines has implanted it: https://github.com/kmwoley/restic-windows-backup