ZFS boot support for bhyveload.

Modelled after the i386 zfsloader. However, with no
2nd stage zfsboot to search for a bootable dataset,
attempt a ZFS boot if there is more than one ZFS
dataset found during the disk probe.

sys/boot/userboot/zfs
 - build the ZFS boot library

sys/boot/userboot/userboot/
 conf.c
  - Add the ZFS pool and filesystem tables
 devicename.c
  - correctly format ZFS devices
 main.c
  - increase the size of the libstand malloc pool
  to account for the increased usage from ZFS buffers
  - probe for a ZFS dataset, and if one is
  found, attempt to boot from it.

usr.sbin/bhyveload/bhyveload.c
 - allow multiple invocations of the '-d' option
 to specify multiple disks e.g. a raidz set.
 Up to 32 disks are supported.

Tested with various combinations of GPT, MBR, single
and multiple disks, RAID-Z, mirrors.

Reviewed by:	neel
Discussed with:	avg
Tested by:	Michael Dexter and others
MFC after:	3 weeks