Add dracut support

Add dracut support

To simplify the process of using zfs as your root filesystem a
zfs-drucat sub-package has been added.  This sub-package adds a zfs
dracut module which allows your initramfs to be rebuilt with zfs
support.  The process for doing this is still complicated but there
is clearly interest from the community about getting this working
well and documented.  This should help lay some of the groundwork.

Longer term these changes should be pushed in the upstream dracut
package.  Once that occurs this subpackage will no longer be
required for new systems, however we may want to conditionally
build this package in the future for systems running older
dracut versions.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>