dual_iso: tell enabled from active & add toggle function

Created originally on Bitbucket by Marsu42

Was already merged in Bitbucket before import, marked as merged by the import user

Source changeset not longer availableat the time of import. Commit listwill look empty

This lets other modules check if the user has explicitly enabled/disabled the module (and not if the module just happens to enable itself) and adds a way to let other modules toggle dual_iso on or off. Alex: Feel free to nitpick as it affects the core and your modules.

Background: one of my (upcoming :-)) modules adds an ingenious way to quickly toggle dual_iso by setting the ec to a configurable low value (like say -2) so you only have to turn the dial instead of entering the menu. For this to work, the recovery iso must be higher of course like iso 100/400 so you can expose for the highlights.