Add FreeBSD 'zpool labelclear' command

The FreeBSD implementation of zfs adds the 'zpool labelclear'
command. Since this functionality is helpful and straight
forward to add it is being included in ZoL.

References:

freebsd/freebsd@119a041dc9230275239a8de68c534c0754181e7e

Ported-by: Dmitry Khasanov <pik4ez@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1126