Enhancement: Add extended options to configure Windows Shadow Copy Service

Previous, restic always used a 120 seconds timeout and unconditionally created
VSS snapshots for all volume mount points on disk. This behavior can now be
fine-tuned by the following new extended options (available only on Windows):

- `-o vss.timeout`: Time that VSS can spend creating snapshot before timing out (default: 120s)
- `-o vss.exclude-all-mount-points`: Exclude mountpoints from snapshotting on all volumes (default: false)
- `-o vss.exclude-volumes`: Semicolon separated list of volumes to exclude from snapshotting
- `-o vss.provider`: VSS provider identifier which will be used for snapshotting

For example, change VSS timeout to five minutes and disable snapshotting of
mount points on all volumes:

    restic backup --use-fs-snapshot -o vss.timeout=5m -o vss.exclude-all-mount-points=true

Exclude drive `d:`, mount point `c:\mnt` and a specific volume from snapshotting:

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

Uses 'Microsoft Software Shadow Copy provider 1.0' instead of the default provider:

    restic backup --use-fs-snapshot -o vss.provider={b5946137-7b9f-4925-af80-51abd60b20d5}

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