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

src/test/test-dev-setup.c

@@ -20,7 +20,6 @@ int main(int argc, char *argv[]) {
         f = prefix_roota(p, "/run");
         assert_se(mkdir(f, 0755) >= 0);
 
-        f = prefix_roota(p, "/run/systemd");
         assert_se(make_inaccessible_nodes(f, 1, 1) >= 0);
 
         f = prefix_roota(p, "/run/systemd/inaccessible/reg");