bug#56791: [PATCH] Add window arg to splitting commands, new commands to split root window

[Juri Linkov]

> But need to be careful with adding 'window-to-split' arg to 
> 'split-window-below'.
> Its split-window-keep-point=nil logic might not be applicable to some windows.

When 'split-window-keep-point' is customized to nil, then the second call of
'split-window-below' fails with

  Debugger entered--Lisp error: (wrong-type-argument stringp nil)
    split-window-below(nil #<window 6>)
    split-root-window-below(nil)
    funcall-interactively(split-root-window-below nil)
    command-execute(split-root-window-below)