bug#12253: Restore window start position

[Juri Linkov <juri@jurta.org>]

This is a request for improvement initiated in bug#12230 and
continued here in a new report.  The request is to restore the original
window start position when returning back to the previous node.

Martin writes in bug#12230:

>> +	  (set-window-start (selected-window) (nth 3 hist))))))
> [...]
>> +    (set-window-start (selected-window) ostart)))
> [...]
>> +    (set-window-start (selected-window) ostart)))
>
> Are you sure you want to force start positions here?

The first change in `Info-find-node-2' is needed to avoid recentering
when the target Info node does not exist and the reader stays in the
same node when the error message is reported in the echo area as
"No such node or anchor".  (What is interesting here is that this
change is needed only when the link leads to another Info file.
But when the link leads to another node of the same Info manual,
this change is not needed, because the window position stays the same.)

The second change in `Info-history-back' and in `Info-history-forward'
is to keep the original window positions while navigating through
the Info history backward and forward.  I think this has the same importance
as keeping the original window positions while navigating buffers
with `next-buffer' and `previous-buffer'.