instances: Refactor so that default values and failures are handeld better.

Now:
- for each list of possible component values, it is specified whether
there is default value (last) and whether user should be warned if selection
of one components shadows others (such as POR vs. fairness for DVE).
- user is warned is component would be selected but is is not compiled in
(such as POR when compiled with O_SMALL, or missing generator)