Enhancement: Add support for Volume Shadow Copy Service (VSS) on Windows

Volume Shadow Copy Service allows read access to files that are locked by
another process using an exclusive lock through a filesystem snapshot. Restic
was unable to backup those files before. This update enables backing up these
files.

This needs to be enabled explicitely using the --use-fs-snapshot option of the
backup command.

https://github.com/restic/restic/issues/340
https://github.com/restic/restic/pull/2274
