Add bunch of new functionality to lpreserver

Replace "mirror" with "zpool" sub-cmd, which will allow you to
attach / detach / online / offline / list various zpool drives

* Important notes *

When attaching a new disk to a zpool, the drive will
be wiped and re-formatted with a 'freebsd-zfs' file-system of the same
size as the first-disk in th zpool array.

If the first disk has a swap partition it will be created also,
but not activated, since advanced users can handle that as the see fit.

Last but not least, GRUB will be stamped on the new disk, making it
bootable should another drive in the array go bad.