Add new functions for splitting the root window

Add new functions for splitting the root window

* lisp/window.el (split-window-right): Add optional argument to
control which window is split (previously, would only split selected
window).  Update docstring.
* doc/lispref/windows.texi (Splitting Windows): Update docs for
`split-window-right'.
* lisp/window.el (split-window-below): Add optional argument to
control which window is split (previously, would only split selected
window).  Update docstring.
* doc/lispref/windows.texi (Splitting Windows): Update docs for
`split-window-below'.
* lisp/window.el (ctl-x-map): Bind `split-root-window-right' to 9 in
ctl-x-map.  This is consistent with binding other window-splitting
operations to numbers in this map.
* lisp/window.el (ctl-x-map): Bind `split-root-window-below' to 7 in
ctl-x-map.  This is consistent with binding other window-splitting
operations to numbers in this map.
* lisp/window.el (split-root-window-right): New function to split
whole frame.
* doc/lispref/windows.texi (Splitting Windows): Add documentation for
`split-root-window-right'.
* lisp/window.el (split-root-window-below): New function to split
whole frame.
* doc/lispref/windows.texi (Splitting Windows): Add documentation for
`split-root-window-below' (bug#56791).