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

[martin rudalics]

> But how would the user override this behavior in a way that can depend
> on the buffer being shown (i.e. via special-display-buffer-names)?

Customizing `pop-up-windows' would give (1) a general preference where
and how `display-buffer' should pop up windows, and (2) a possibility to
override decisions made by applications like dired on where and how to
pop up windows.

`special-display-buffer-names' and `special-display-regexps' would be
handled as usually, before checking `pop-up-windows'.  If we want, we
can provide things like 'window-below or 'window-at-bottom as phony
parameters as suggested earlier.

All this is also a question of where and how to document behavior best.
I tried to understand and document the special-display stuff but am
pretty sure that I still haven't got it right yet.  I strongly doubt
that anyone but you really knows how this is supposed to work.

OTOH `pop-up-windows' has a one line doc-string, so I thought we could
try that first.

martin