bug#13594: Planning Emacs-24.4

[martin rudalics]

> Implemented as requested. I have also briefly tested with ggtags and it
> worked. Comments welcome. Thanks.

Why didn't you implement the may-fail part?  IIUC `display-buffer'
should *not* return nil unless may-fail has been set.  So if may-fail is
not set, `display-buffer' should either create a new window, a new
frame, or reuse some window at any cost.

And there's no use for `display-buffer' returning t.

martin