zhack: Add repair label option
In case if all label checksums will be invalid on any vdev, the pool
will become unimportable. The zhack with newly added cli options could
be used to restore label checksums and make pool importable again.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Fedor Uporov <fuporov.vstack@gmail.com>
Closes #2510
Closes #12686