machined: introduce per-user instance (#39123)

This is the first part of #38728, just the machined stuff, no the
importd stuff.

This definitely makes sense of its own, hence let's get this in first.

The original PR contains a tescase that tests machined + importd in
combination. This PR here hence is without a testcase, but it's there,
just in the other PR.

This looks large and is large, but do note that much of the machined
changes are very repetitive: they conditionalize PK checks to the system
version, as PK doesn't make sense in the use rversion.