bug#6204: vc-dir always splits the frame

[martin rudalics]

> Yes, I'm picturing the special arg as (conceptually) two alists, and
> that we'd combine with the user's own setting via something like
>
>   (append override-params
>           (lookup special-display-regexps)
>           default-params)

Good.  Now how do `pop-up-windows', `pop-up-frames', and
`display-buffer-reuse-frames' fit into this?  I suppose these are the
values for all buffers not mentioned by `special-display-regexps'.  What
if `pop-up-windows' is nil and "default-params" indicates that the
application wants to pop up a window (but does not insist).  Probably
`pop-up-windows' should prevail in this case.

But maybe we could do away with `pop-up-windows' and friends by adding a
default match-any-name expression to `special-display-regexps'?  This
would simplify doc-strings considerably.

martin