Make the installer clear old pool labels from new partitions

When disks are reused, old pool labels on the disk may be revealed when new partitions are added. This can potentially cause problems booting and importing pools.

Attempt to clear existing pool labels whenever a new partition is added.

This is done by running `zpool labelclear -f /dev/$diskpart` after adding a new partition to a disk and should be done for all partition types, not just freebsd-zfs.

The user will have already agreed to wipe the contents of the disk by the time we have repartitioned it.