warning message control: jumbo patch so far

[DJ Delorie]

> In order not to hold it indefinitely, I would like you apply it but

Done.

> follow up with a patch

>   * that adds a field "show_options" to diagnostic_context, that
>     consistently play the role of diagnostic_show_options;
>   * that has this
> 
> | +  if (opt && ! option_enabled (opt))
> | +    return;
> | +
>                    done by report_diagnostic.  That for example,
>     ensures that any diagnostic kind (error, warning, note, ...) has
>     it checked properly.

Well, one of our customers wants to be able to silence certain
pedantic errors, so I'm working in this direction anyway ;-)

pedwarn() will eventually have to have the extra option_index
parameter too, perhaps error() et al also.