[Simh] Write sector headers to disk

[Lars Brinkhoff]

David Brownlee wrote:
> How about storing them only in memory - still an incomplete
> implementation, but should be enough to pass the SALV tests...

I should clarify that to pass the tests, only the sector data needs to
be written.  Rather than ignored as now.

SALV *also* uses the two key fields in the header words.  Those are not
part of the verification phase.  I'm not yet sure to what extent the key
fields are necessary or not.