new feature: mount of directory tree

[tomberek]

A mount-tree or mount-tar (or just mount?) would replace casync mount. Reusing the sparse mount support would be great.

[folbricht]

Mounting catar files is probably the one significant feature missing in desync compared to casync. It's doable but non-trivial and I need to find the time to actually tackle that one. Will look into it, but it won't be a quick thing to implement.