Bugfix: Restore symbolic links on windows

We've added support to restore symbolic links on windows.
Because of windows specific restrictions this is only possible when running
restic having SeCreateSymbolicLinkPrivilege privilege or when running as admin.

https://github.com/restic/restic/issues/1078
https://github.com/restic/restic/issues/2699
https://github.com/restic/restic/pull/2875
