Add another epicycle to -A processing that ensures ONLY the

Add another epicycle to -A processing that ensures ONLY the
partition table is changed. Not the GPT header. Not the MBR. And
only write back as much partition table information as the header
claims to have room for.

At a minimum should make -A safer when operating on the Apple M1
GPT.

A major overhaul of this code is urgently needed before someone
sneezes too hard in its vicinity.

Feedback kettenis@ & ok deraadt@