(Fdisplay_buffer): Move call to

(Fdisplay_buffer): Move call to
Vsplit_window_preferred_function out of conditions that check
if window is eligible for vertical splitting.
When Vsplit_window_preferred_function is non-nil, call it and use
its non-nil return value as window.  Otherwise, continue doing
vertical splitting using Fsplit_window with arg horflag=nil.
(syms_of_window) <Vsplit_window_preferred_function>: Change the
default value from `split-window' to nil.