vss: Implement VSS support for Windows

The VSS support works for 32 and 64-bit windows, this includes a check that
the restic version matches the OS architecture as required by VSS. The backup
operation will fail the user has not sufficient permissions to use VSS.

Snapshotting volumes also covers mountpoints but skips UNC paths.

changelog/unreleased/issue-340

@@ -0,0 +1,12 @@
+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