bug#3454: <SPC> when reading the emacs manual sometimes goes to thewrong node

[Drew Adams]

> "recent" versions of makeinfo label index nodes with an
> "index cookie", so if we see such a cookie, we know for sure 
> we're in an index node.  That should make it possible to write
> a good patch.

I think this bug should be fixed, no matter how minor it might seem to someone
who understands well the structure of Info or its implementation. It really
should be possible for someone to use a single key (`SPC') to traverse the
manual, with no exceptions.

Even adding a pseudo menu item at the beginning of the index, which would DTRT
(since SPC goes to the first menu item's node), would be useful, if no better
solution can be found. I'm not necessarily proposing such an ugly hack, but it
might be better than nothing.

`n' doesn't go anywhere when there is no `Next' node, and `SPC' doesn't go to
the right place when you are at the end of an index. Users should not need to
sometimes use `n' and sometimes use `SPC', and think consciously about which
might be appropriate and work, in order to simply traverse the manual from front
to back (and then wrap back to the front).

And this attitude is not a great one, IMO:

"Morale: do not use SPC at the end of an index, use `n'.
I'm closing this report."