bug#6204: vc-dir always splits the frame

[martin rudalics]

>> How would you proceed?
>
> 1- fix all the Emacs code that modifies those vars.

IMHO the only way to do that is to (1) provide a new set of variables
Elisp code _is_ allowed to change and (2) have `display-buffer' use
these iff the respective buffer has no corresponding entry in the
special-display-... options.  If I do that and a buffer is now in the
(same-window . t) group of `special-display-regexps', how should
`Buffer-menu-switch-other-window' proceed with that buffer?  Bind
`special-display-regexps'?  Call `switch-to-buffer-other-window'?  So
far the special-display-... options had an excuse: If this variable
appears \"not to work\", ...

> 2- declare the variable obsolete.
> 3- Tada!

martin