inaccessible: move inaccessible file nodes to /systemd/ subdir in runtime dir always

Let's make sure $XDG_RUNTIME_DIR for the user instance and /run for the
system instance is always organized the same way: the "inaccessible"
device nodes should be placed in a subdir of either called "systemd" and
a subdir of that called "inaccessible".

This way we can emphasize the common behaviour, and only differ where
really necessary.

Follow-up for #13823