bug#26396: 25.1; char-displayable-p on a latin1 tty

[Eli Zaretskii <eliz@gnu.org>]

> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Sun, 16 Apr 2017 20:26:01 -0700
> 
> Kevin Ryde wrote:
> 
> > Debian has an easy line in /etc/default/console-setup.  The default is
> > now utf8 but there's lots more.  I have it latin1.
> 
> In that case I stand corrected: some people are stull using non-UTF-8 Linux 
> consoles. I don't know of any convenient programmatic way for Emacs to determine 
> whether the console is in UTF-8 output mode, though. (I can think of complicated 
> ways, involving outputting bytes to the screen and seeing what happens to the 
> cursor position; but this would be destructive to the screen contents.) I agree 
> Emacs shouldn't be changing the mode.

I agree.

Regarding testing it, the simplest way is to provide a user variable
which will tell Emacs about the console's mode, and ask users who
don't want the UTF-8 mode to set that variable to that effect.  Or
maybe my idea about using terminal-coding-system as an indication of
that could be workable, in which case it's a better alternative.
WDYT?

I can offer help in reviewing the patches and perhaps also writing
some of that, but I cannot test the code, as I don't have a convenient
access to a Linux console where I could run Emacs I build.

TIA