`opt.solution_prefix` & `opt-dump_models` not working

[bruderj15]

Hello.

When playing around with the optimizer I stumbled on similar issues as in #1463 .
On Ctrl-C responding with unknown and the best known intermediate solution unfortunately does not yield any value.
Why can't it be sat instead?
Although not optimal, the model clearly is supposed to satisfy the constraints.

When trying to write intermediate solutions or dumping intermediate models nothing happens.
Should be reproducible with this: foo.txt

Version: 4.13.0

[NikolajBjorner]

The option :opt.solution_prefix had been bit-rotting. It wasn't enabled.
I have re-enabled it and put it in a place where the models get dumped to files.
There are some other ways to retrieve models:

• :opt.dump_models = true - prints the models to the console
• Using the programmatic API you can attach a callback when the solver has a model.

I would prefer to return unknown even if there is a model in this case to distinguish optimality from feasibility in the optimization scenario. The optimal remains "unknown".