When ignoring parameters let 'split-window' split root window (Bug#76317)

When ignoring parameters let 'split-window' split root window (Bug#76317)

This fixes a behavior introduced by the fix of Bug#73527 that splits the
main window when the caller asks for splitting the root window.

* lisp/window.el (window--make-major-side-window): Bind
'ignore-window-parameters' to t so 'split-window' won't mess
with the WINDOW argument when it specifies the root window.
(split-window): If 'ignore-window-parameters' is non-nil, don't
try to split the main window instead of the root window.

(cherry picked from commit 310f62a9489b9c00d5e60d2ab16e7dc54afcadb9)