[RFC] Implement -list-thread-groups.

[Joel Brobecker]

> Since Michael commented on the following assertion:
> 
> > +  gdb_assert (requested_thead == -1 || pid == -1);
>                            ^^^^^ Ooops, typo???
> 
> I think it's important to explain it in the function description.
> Can you add a line or two explaining at the end of the function
> description explaining that it doesn't make sense to provide both
> the pid and the thread at the same time, and thus at least one
> of the two must be set to 1.

BTW: I should have said that I'm neutral to what the function should
do in case the two parameters are not -1. I'll let you guys decide
what to do in that case, and I'm fine with doing that as a followup
patch (the consequences of this decision are not immediately visible,
I believe).

-- 
Joel