bug#6204: vc-dir always splits the frame

[martin rudalics]

>> I thought the idea was that an application can override a user's
>> customizations iff it explicitly states that.
>
> Yes and the users can still do that with special-display-regexps.
> But pop-up-frames and pop-up-windows are really very vague global
> preferences, whereas an application going through to trouble of
> specifying default-params has decided that it usually deserves
> special treatment.

So we apparently want to support options like `pop-up-windows' and
`pop-up-frames' forever.  The proposed behavior from strongest to
weakest is therefore

(1) An overriding argument set by the application.

(2) The user setting for `special-display-regexps' (including regexps
    matching _any_ buffer).

(3) A non-overriding argument set by the application.

(4) Vague global preferences like `pop-up-windows'.

Shall applications continue to bind `pop-up-windows' and `pop-up-frames'
as they do now?  I suppose where they do now they should use (1) or (3)
instead.

> Some of the intention behind the `default-params' would be to get rid of
> calls to switch-to-buffer (replaced by calls to pop-to-buffer with
> a same-window default-param).  Such cases already disregard
> pop-up-windows and pop-up-frames, so ignoring them w.r.t default-params
> would be consistent with such a case.

You mean all cases where the buffer argument of `switch-to-buffer' names
an existing buffer (probably all of them do, but I didn't check).

martin