homed: always align home file systems to 4K boundaries

Let's carefully align all home file systems to 4K sector boundaries.
It's the safest thing to do, to ensure good perfomance on 4K sector
drives, i.e. today's hardware.

Yes, this means we'll waste 3.5K when resizing home dirs, but I think we
can live with that.

This ensures both the offsets where we start and the sizes of the file
systems/partitions/disk images are multiples of 4K always, both when
creating a new image and when resizing things.

Note that previously we aligned everything to 1024, but weren't quite as
careful.