core/systemctl: when switching root default to /sysroot/

core/systemctl: when switching root default to /sysroot/

We hardcode the path the initrd uses to prepare the final mount point at
so many places, let's also imply it in "systemctl switch-root" if not
specified.

This adds the fallback both to systemctl and to PID 1 (this is because
both to — different – checks on the path).