##### Implement a new unionfs
* unionfs is a particularly useful pseudo-fs which allows to have an upper and a lower filesystem on a single mountpoint. The upper mountpoint is mostly transparent, so that the lower mountpoint is accessible.
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* A typical use case is mounting a tmpfs filesystem as the upper and a read-only FS as the lower mp. This way files can be edited transparently even on a RO filesystem without actually modifying it.
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* The current unionfs is completely broken as it relies on the whiteout VFS technique which is not supported by HAMMER.
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