Use systemd socket for non-root devices

[kelderek]

root device needs to be unlocked by initrd and use the keyscript parameter. non-root devices are handed off to systemd by default, which doesn't support keyscript directly. Can use a keyfile that is actually an AF_UNIX socket backed by a systemd service.

script should detect non-root devices and use that method instead of the initramfs parameter which tells initrd it should mount the devices