D051 - The two ways of writing a 1 near the end of the superblock are two fields, not one

An image with a seed writes an index at 0x36C and the seed after it. An image without one writes 1 at 0x368 and stops. Both had been measured as "a 1 near the end of the header", and reading could never tell them apart because nothing reads either.

The writer is what separated them, and it is the second time in this area that writing has found something reading could not (D042 was the first). Both offsets are now named. The inner filesystem is unseeded and uses NO_SEED_INDEX; the outer one is seeded and uses UNKNOWN_INDEX.

The same pass corrected N_BLOCK. It is not the block count, despite the name: the source writes a literal 1 and remarks that it always is. Writing the real count there would have been more plausible and would have made every image this builds differ from every image examined, in a field nothing checks.