bug#3540: Please reserve a ctrl-key combination for interoperability

[Drew Adams <drew.adams@oracle.com>]

> As a partial solution, the manual *might* suggest to use C-z for that,
> especially that is is bounded to a 99.99% useless command by default
> (and using e.g. screen or tmux makes it 100% useless).

No, please don't do that.  IMO:

`C-z' is better used by users and libraries as a
_prefix key_ (by users who are willing to forego
the default binding).

The manual should not suggest that users bind any
particular keys.  It's OK for a 3rd-party library
to suggest key bindings.  It's not good for Emacs
itself to do that.

3rd-party libraries are opt-in by users.  Using
one is like adding its feature/code to your init
file - it's a user choice.

The same isn't true of much of the code distributed
by Emacs.  And even when a distributed library (e.g.
`dired-x.el') is opt-in, Emacs should not suggest
bindings for its commands.  "Suggestion" by Emacs
is sometimes mistakenly taken by users as a "rule"
or a convention.

There's no good reason for Emacs to suggest that
users use `C-z' for anything particular.