Optionally avoid that switching to previous or next buffer shows that buffer twice.

Optionally avoid that switching to previous or next buffer shows that buffer twice.

* window.el (switch-to-visible-buffer): New option.
(switch-to-prev-buffer, switch-to-next-buffer): Observe
switch-to-visible-buffer.  Make sure that checking for a window
showing a buffer already is done on the same frame.

* windows.texi (Window History): Describe new option
switch-to-visible-buffer.