[$] Filesystem UID mapping for user namespaces: yet another shiftfs

The idea of an ID-shifting virtual filesystem that would remap user andgroup IDs before passing requests through to an underlying real filesystemhas been around for a few years but has never made it into the mainline. Implementationshave taken the form of shiftfs and shifting bind mounts. Now there is yetanother approach to the problem under consideration; this one involvesa theoretically simpler approach that makesalmost no changes to the kernel's filesystem layer at all.