squash: also squash systemctl if switch-root is not needed

squash: also squash systemctl if switch-root is not needed

systemctl need to be accessible on switch-root, but we unmount the
squash image on switch-root, so it will fail. systemctl depends on a lot
of libraries, squash them can save more RAM. So allow modules
(eg. kdump) to tell dracut that switch-root will be intercepted,
then we don't need to take care of that.

Signed-off-by: Kairui Song <kasong@redhat.com>