Enhancement: Add options to configure Windows Shadow Copy Service

Restic always used 120 sec. timeout and unconditionally created VSS snapshots
for all volume mount points on disk. Now this behavior can be fine-tuned by
new options, like exclude user specific volumes and mount points or completely
disable auto snapshotting of volume mount points.

For example:

    restic backup --use-fs-snapshot -o vss.timeout=5m -o vss.excludeallmountpoints=true
    
changes timeout to five minutes and disable snapshotting of mount points on all volumes, and

    restic backup --use-fs-snapshot -o vss.excludevolumes="d:\;c:\mnt\;\\?\Volume{e2e0315d-9066-4f97-8343-eb5659b35762}"

excludes drive `D:`, mount point `C:\MNT` and specific volume from VSS snapshotting.

    restic backup --use-fs-snapshot -o vss.provider={b5946137-7b9f-4925-af80-51abd60b20d5}
    
uses 'Microsoft Software Shadow Copy provider 1.0' instead of the default provider.

https://github.com/restic/restic/pull/3067