bug#8789: 23.3; debug backtrace buffer changes window on step-through

[martin rudalics]

> Only one wish remains: it is not possible to resize the height of the
> window displaying the *Backtrace* buffer durably.  When I resize it with
> the mouse in emacs -Q, this is undone when I hit d in the debugger.  It
> would be cool if the size of the window could be changed durably.

I committed a fix.  Please try it.

martin