machined: expose "UID shift" concept for containers

UID/GID mapping with userns can be arbitrarily complex. Let's break this
down to a single admin-friendly parameter: let's expose the UID/GID
shift of a container via a new bus call for each container, and let's
show this as part of "machinectl status" if it is not 0.

This should work for pretty much all real-life full OS container setups
(i.e. the stuff machined is suppose to be useful for).  For everything
else we generate a clean error, clarifying that we can't expose the
mapping.

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