[ENBD] md5sum switch

[Peter T. Breuer]

"Also sprach Arne Wiebalck:"
> I didnt't mean you to spend your whole weekend to reply my request!

I needed to think about this, and so I needed to try various
solutions.

I've recently hit on the happy idea of replacing "md5_enable" with
"md5_noauto", so that the default will be the automatic switching
that happens as present, and when noauto is set, then the current
state of md5sum will always be preserved, thus allowing changes to
md5sum state to be made externally and respected.

Peter