MFC r297763: MFV r297760: 6418 zpool should have a label clearing command

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

Closes #83
Closes #32

openzfs/openzfs@9663688425131744221ea99f9e66b9ed964492ae

FreeBSD already had zpool labelclear functionality, so this is mostly
just a diff reduction.