add 'growpart' function.

[smoser]

If you "grow" a disk and boot the system, disko seems unaware, but it really ticks off other tools.

I did an install, then attachd the disk as larger than it was.
Then did an add-partition with disko. I'm not sure what happened, but parted really was upset.

It'd be good to have a consistent story here.
One of

• re-write the partition table with the end in the correct place
• keep the old one