[m-rev.] for review: make --warn-unused-imports the default

[Zoltan Somogyi]

On Thu, 19 Jan 2017 10:00:05 +1100 (AEDT), Julien Fischer <jfischer at opturion.com> wrote:
> > In some rare cases, an import is needed only in some circumstances
> > (such as grades),
> 
> "rare" is a relative thing; it's common enough in code that has to work
> with multiple backends and that uses a significant amount of foreign
> code.

Agreed, but it is rare in the Mercury system; e.g. only a small fraction of
the compiler's modules contain any foreign language code at all.
That is why the diff turned the option on for our system, instead of
just changing the default in options.m.

Zoltan.