MFC r224171 (gibbs):

MFC r224171 (gibbs):
Add the "zpool labelclear" command.  This command can be
used to wipe the label data from a drive that is not
active in a pool.  The optional "-f" argument can be
used to treat an exported or foreign vdev as "inactive"
thus allowing its label information to be cleared.

git-svn-id: svn://svn.freebsd.org/base/stable/8@229570 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f