6418 zpool should have a label clearing command

illumos/illumos-gate@6401734d545a04c18f68b448202f9d9a77216bb9
https://github.com/illumos/illumos-gate/commit/6401734d545a04c18f68b448202f9d9a77216bb9

https://www.illumos.org/issues/6418

An easy, direct means of sanitizing pool vdevs can be helpful for management
purposes.
FreeBSD has had a 'zpool labelclear' for some time, see: https://
svnweb.freebsd.org/base?view=revision&revision=224171
SpectraBSD has a slightly updated version, which I propose for inclusion.

Reviewed by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Will Andrews <will@firepipe.net>

Note: the bulk of the change has been already imported, this is a follow
up that imports zpool.1m changes.