[$] ID mapping for mounted filesystems

Almost every filesystem (excepting relics like VFAT) implements the conceptof the owner and group of each file; the higher levels of the operatingsystem then use that information to control access to those files. Fordecades, it has usually sufficed to track a single owner and group for eachfile, but there is an increasing number of use cases wanting to make thatownership relative to the environment any given process is running in.Developers have been working for a few years to find solutions to thisproblem; the latest attempt is the ID-mappedmounts patch set from Christian Brauner.