Add -blackbox flag to cutpoint pass

[RCoeurjoly]

What are the reasons/motivation for this change?
Black/white boxes cannot be processed by sby, so they must be substituted by $anyseq cells.
Explain how this is achieved.
Adding a -blackbox flag to cutpoint.
If that flag is set, we find all modules that are black/white boxes, add their module names to the select pattern, and remove their black/whitebox attribute.
If applicable, please suggest to reviewers how they can test the change.

[nakengelhardt]

This approach won't work. extra_args can throw a message about invalid arguments, and the design must not be modified in any way prior to that. And given that the select argument is a stack-based sequence of instructions, appending something to the user-provided argument can have quite unpredictable and difficult-to-debug consequences. Please make a separate loop in the body of the pass that handles modifying the blackboxes directly.

[RCoeurjoly]

This approach won't work. extra_args can throw a message about invalid arguments, and the design must not be modified in any way prior to that. And given that the select argument is a stack-based sequence of instructions, appending something to the user-provided argument can have quite unpredictable and difficult-to-debug consequences. Please make a separate loop in the body of the pass that handles modifying the blackboxes directly.

Yes, I think the approach followed in #4565 could be more succesful.
It takes care of not breaking passes that need to filter out blackboxes and at the same time enables blackbox selection when needed.