Modify `infer_meta_predicate/2` such that `?` is reported rather than `*`

[eshelyaron]

This PR makes infer_meta_predicate/2 consistent with the general preference of SWI-Prolog for using ? rather than * as the unspecified argument mode specifier.

While this is a breaking change, as far as I can tell library(prolog_metainference) is only used by library(prolog_codewalk) which does not rely on the current behavior of infer_meta_predicate/2 using *, and I couldn't find any public code outside of SWI-Prolog itself currently using infer_meta_predicate/2.