dual_iso: make dr gained a setting for auto-disabling

Created originally on Bitbucket by Marsu42

Changesets for this Pull Request have not been imported, because it had been already declined on Bitbucket. Marked as closed by the import user.

In essence, this change I already suggested in the forum lets me limit dual_iso to lower base iso values and use auto iso w/o constantly switching dual_iso on and off.

I also took the liberty to replace absolute numbers with the new macros from lens.h, poor /me has a hard time understanding where these 72 and esp. 8 numbers originate from :-o