#143 leftovers and followups

[Villemoes]

Some things off the top of my head that should/could be done at some point after #143 is merged, but which I'd rather not include in that already-way-too-long series.

• check that no partition in the partition table lives before the GPT array (essentially redoing image-hd: enforce no GPT partition before the primary Partition Entry Array #125)
• extend insert_image so it can be used as a full replacement for the few remaining uses of pad_file
• perhaps add a check that when hd->gpt and image size is given that it's a multiple of 4K, to be consistent with the case where the image size is auto-computed to place the end of the gpt backup at a 4K boundary.
• some code cleanups might be in order - I ended up introducing helpers like partition_end and min_ull/max_ull, and some existing code might benefit from using those.