bug#6616: S-TAB is mismapped in the *Help* buffer

[Drew Adams]

> > I say, pick a solution and make it consistent across the modes
> > shipped with Emacs.  IMHO, [S-tab] is the 'correct' binding,
> > since we press those actual keys.
> 
> In most cases I've seen `backtab' is the "correct" binding 
> because it's
> the one that makes logical sense: the command is about doing something
> in the opposite direction from what the `tab' key does.
> The reason why S-tab is (also) used is that `backtab' is rarely
> available, and as a convention we use S-tab as a poor man's backtab.

Hm. Then perhaps I do disagree.  I thought that I had misunderstood and this was
not, after all, about binding the S-tab or backtab key to any given command by
default, but it was just about linking S-tab to backtab (both being keys).

I would oppose binding either key to some command everywhere, "fallback" or not.
As soon as that happens we will get people who complain if a 3rd-party library
binds S-tab, even optionally or via an initialization command.

S-tab and backtab keys can be linked to each other by default for convenience,
but they should both be free of any command by default - except in particular
contexts (e.g. *Help*, Info).

BTW, there are not only the S-tab and backtab keys that should (perhaps) be
linked together by default, but also the S-iso-tab key.  Typically, when code
binds S-tab it also needs to bind S-iso-tab.  At least in some cases, we have
already taken care of that via backtab, IIRC.