switch-root: automatically make target switch root dir a mount point

switch-root: automatically make target switch root dir a mount point

Let's make sure implicitly that the target directory is a mount point,
instead of doing so manually beforehand. This allows us to drop this
step from the transition into the /run/initramfs/ dir at shutdown.

During the initrd→host transition the switch root operations so far
where towards pre-existing mount points, but there are cetrainly
usecases where it might make sense to siwtch into arbitrary
subdirectories, too.