* Reorganize four-way if-elsif-elsif-elsif as nested if-elses
* Mark unused branch in _make_posargs
`names_with_default` is never `NULL`, even if there are no names with
defaults. In that case it points to a structure with `size` zero.
Rather than eliminating the branch, we leave it behind with an `assert(0)`
in case a future change to the grammar exercises the branch.