Facultatively ignore margins when splitting and resizing windows (Bug#24193)

Facultatively ignore margins when splitting and resizing windows (Bug#24193)

Provide a new window parameter 'min-margins' which allows to
ignore the actual widths of a window's margins when splitting or
resizing that window horizontally.  This should serve as a
workaround for handling the problems raised by Bug#24193.

* lisp/window.el (window--min-size-1): Handle new window
parameter 'min-margins'.
(split-window): Fix text of error messages.
* doc/lispref/windows.texi (Window Parameters): Describe new
parameter 'min-margins'.