Enhancement: Support restoring symbolic links on Windows

The `restore` command now supports restoring symbolic links on Windows. Because
of Windows specific restrictions this is only possible when running restic with
the `SeCreateSymbolicLinkPrivilege` privilege or as an administrator.

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