Ensure that the '-i' command line option and the 'reinit [mbr]'

Ensure that the '-i' command line option and the 'reinit [mbr]'
editing commnd completely remove any existing on-disk GPT header
when writing the MBR.

'-ig'/'reinit gpt' must be used to create GPT format.

Previous interactions between '-i', '-b' and overly clever
'reinit' produced surprising, inconsistent and incorrect
behaviour.

ok deraadt@ kettenis@