Don't kill the *info* buffer in `Info-revert-find-node'.

Don't kill the *info* buffer in `Info-revert-find-node'.
Add `revert-buffer' to defcustom `Info-hide-node-references'.

* lisp/info.el (Info-hide-note-references): Add `:set' tag to `defcustom'
that calls `revert-buffer' on all Info buffers.
(Info-revert-find-node): Remove let-bindings `old-buffer-name',
`old-history', `old-history-forward'.  Add let-binding
`window-selected'.  Remove calls to `kill-buffer',
`switch-to-buffer' and `Info-mode'.  Set `Info-current-file' to nil
before calling `Info-find-node', so `Info-find-node-2' will reread
the Info file.  Restore window positions only when `window-selected'
is non-nil.