Add option to place drive in standby (spin down) after a specific drive has completed its erasure.

[PartialVolume]

In order to conserve power and reduce heat a drive should be placed in standby once it has completed it's erasure.

This is probably more important for systems that are wiping many drives, i.e rack servers with 16-32+ drives that have vastly different sizes of disc, where some drives may finish wiping hours before other drives. Not so relevant if all the drives are much the same size and --autopoweroff can be used to power down the entire system once all drives have completed.