Previously we'd use a directory /run/systemd/nspawn/incoming for
accepting mounts to propagate from the host. This is a bit weird, since
we have a shared namespace: /run/systemd/ contains both stuff managed by
the surround nspawn as well as from the systemd inside.
We now have the /run/host/ hierarchy that has special stuff we want to
pass from host to container. Let's make use of that here, and move this
directory here too.
This is not a compat breakage, since the payload never interfaces with
that directory natively: it's only nspawn and machined that need to
agree on it.
5a27b39518