bug#3142: 23.0.92; split-window-prefered-function should be able to use split-window-horizontally/split-window-vertically

[Stefan Monnier]

> I remember we once considered creating a default function with
> "intelligently splitting" logic (IOW, moving the `else' part of `if'
> in `window--try-to-split-window' to a separate function)
> and setting the default value of `split-window-preferred-function'
> to the name of this function.

Indeed.  IMO, `foo-function' should not accept nil as value and
should be unconditionally called.


        Stefan