Using ZFS as root filesystem

Enable EnterpriseBSD to use a ZFS volume as root filesysem(s).

The following needs to be considere:
 * seeding the /boot/zfs[/zpool.cache] upon creating a new boot filesystem/installation (so that ZFS volumes will be available upon booting from this disk)
 * take care of hostname / hostid change preventing ZFS from initializing the system (export/import with new ID before finishing installation ?) -- http://www.sun.com/msg/ZFS-8000-EY
 * zfs_load="YES" in /boot/loader.conf so that ZFS will be available in kernel when about to mount the rootfs
 * vfs.root.mountfrom="zfs:volname" in /boot/loader.conf so that the kernel will know where to boot from
 * adjusting /etc/fstab to mount the proper ZFS filesystems and not to let fsck check the ZFS mountpoints
 * the default ZFS mountpoint(s) should perhaps remain different than the ones specified in /etc/fstab, to facilitate easy mounting when booting off rescue discs
 * swap space may be created from on a ZFS volume, eventually specifying the "org.freebsd:swap=on" attribute