Cleanup, clarify and generally polish the MBR/GPT initialization

Cleanup, clarify and generally polish the MBR/GPT initialization
code.

'-g' is promoted to be independant of '-i'. This makes it clearer
that there are four mutually exclusive initialization options.

'-i' puts the default MBR on disk, '-g' puts the default GPT on
disk, '-u' updates the MBR boot code on disk and '-A' puts a new
set of GPT partitions on disk without overwriting 'protected'
partitions.

The last initialization option specified is the one executed, so
existing '-i -g' finger memory, etc. continue to work as before.

man page/usage feedback/tweaks from jmc@