Decouple "set confirm" from `from_tty'.

[Joel Brobecker]

> Eventually, I figured out this was caused by having done
> "set confirm off" before the source command.
> 
>  (gdb) source /wrong/path/to/file.py
>  /wrong/path/to/file.py: No such file or directory.
>  (gdb) set confirm off
>  (gdb) source /wrong/path/to/file.py
>  (gdb)
> 
> This was very surprising to me, and I assume to surprise other
> users too.  Turns out that "set confirm off" also has the
> side-effect of running commands with from_tty == 0, which
> silences many things.  E.g.,

I agree this is very surprising! I'm even amazed that we did not
realize this until now....

> So I'd like to stop it doing that.
> Comments?

Sounds good to me. To me, "set confirm off" means do everything
the same except auto-answer "y" when I said to do something that
GDB think might be questionable.

No problem spotted with the patch.

-- 
Joel