bug#6204: vc-dir always splits the frame

[martin rudalics]

> Why not use the existing argument `other-window'
> of `pop-to-buffer' and `display-buffer'?

Let's use it.  Shall we keep the semantics of t as "any window but the
selected one"?  And, shall we rename the argument?

>> Remains the question how to make customizing monsters like
>> `special-display-regexps' simpler.
>
> Very simple: allow users to override the argument `other-window' of
> `pop-to-buffer' and `display-buffer' by the analogous parameter
> `other-window' in `special-display-regexps', e.g.
>
>   ("*vc-dir*" (other-window . t))
>   ("*vc-diff*" (other-window . nil))
>   ("*info*" (other-window . lru))
>   ...

We already have

"As a special case, if FRAME-PARAMETERS contains (same-window . t) ..."

so should we now say

"As another special case, if FRAME-PARAMETERS contains
(other-window . t) ..."

Also I've tried to rewrite the customization interface of
`special-display-regexps' without great success - it's all
too clumsy when the alternatives are nil, t, a list ...

That is, we need (1) a user-friendly doc-string for this and
(2) a user-friendly customization interface.

martin