Remove label

Tried to apply patches available for FreeBSD.

Added zpool labelclear function.
It doesn't work for me, but I can't dig deeper right now.

Please, let you see the patch and maybe you can make it work.

Now it says "Label clear failed on vdev </some/dev>".
Strace says there's some error in lib/libzfs/libzfs_import.c:925 (label_offset).

Please let me know if I can help.