Add the ability to uninitialize

zpool initialize functions well for touching every free byte...once.
But if we want to do it again, we're currently out of luck.

So let's add zpool initialize -u to clear it.

Co-authored-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12451
Closes #14873