MFV r342532: 5882 Temporary pool names

MFV r342532: 5882 Temporary pool names

Note that this commit brings only formatting changes that were done
during the final review of the illumos change, because FreeBSD got the
main changes before illumos.

illumos/illumos-gate@04e56356520b98d5a93c496b10f02530bb6647e0
https://github.com/illumos/illumos-gate/commit/04e56356520b98d5a93c496b10f02530bb6647e0

https://www.illumos.org/issues/5882
  This is an import of the temporary pool names functionality from ZoL:
  https://github.com/zfsonlinux/zfs/commit/e2282ef57edc79cdce2a4b9b7e3333c56494a807
  https://github.com/zfsonlinux/zfs/commit/26b42f3f9d03f85cc7966dc2fe4dfe9216601b0e
  https://github.com/zfsonlinux/zfs/commit/2f3ec9006146844af6763d1fa4e823fd9047fd54
  https://github.com/zfsonlinux/zfs/commit/00d2a8c92f614f49d23dea5d73f7ea7eb489ccf1
  https://github.com/zfsonlinux/zfs/commit/83e9986f6eefdf0afc387f06407087bba3ead4e9
  https://github.com/zfsonlinux/zfs/commit/023bbe6f017380f4a04c5060feb24dd8cdda9fce
  It is intended to assist the creation and management of virtual machines
  that have their rootfs on ZFS on hosts that also have their rootfs on
  ZFS. These situations cause SPA namespace collisions when the standard
  name rpool is used in both cases. The solution is either to give each
  guest pool a name unique to the host, which is not always desireable, or
  boot a VM environment containing an ISO image to install it, which is
  cumbersome.

MFC after:	1 week
Sponsored by:	Panzura