Fix M-x visit-menu

* info/nodes.c (info_copy_node): New function, with code split
from info_split_window.
* info/session-cmd.c (info_split_window): Call info_copy_node.
(info_visit_menu): Rewrite to use info_copy_node.  Do not crash
if enough new windows could not be created for all the referenced
nodes.

Crash with "M-x visit-menu" reported by Aidan <aidanczhai@gmail.com>
for "info texinfo".