[CFR][ZFS] Add "zpool labelclear" command.

[Per von Zweigbergk]

We here at IT-assistans have found the ability to clear ZFS labels very useful for testing. We have an internal utility written in-house to clear ZFS labels. It's rather dumb and dangerous which is why I have chosen not to publish it. (Oh, the irony of being held accountable for a utility designed to cause data loss causing data loss.) Having a labelclear command with some extra molly guards on it in the normal userspace ZFS commands would be useful to us at least for testing.