bug#1806: dired-pop-to-buffer in wrong place

[martin rudalics]

> Yes, it works perfectly for Dired when `split-width-threshold' is nil.

Fine.

> But the problem is that I prefer a wide-screen configuration with
> horizontally split windows, so I can't set `split-width-threshold' to nil.
>
> There are a few exceptions where obeying non-nil `split-width-threshold'
> is not desirable.  One exception is displaying a list of files in Dired,
> and another is displaying the Calendar window.  Maybe there are a few
> of others.

So the only thing we have to decide is whether we want to hardcode this
in `dired-pop-to-buffer' (and for the Calendar window and others) or
make it optional.

martin