Amend Dracut module to export ZFS root on shutdown

Make use of Dracut's ability to restore the initramfs on shutdown and
pivot to it, allowing for a clean unmount and export of the ZFS root.
No need to force-import on every reboot anymore.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
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