bug#20545: New minor mode Electric Punct

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Eli Zaretskii <eliz@gnu.org>,  20545@debbugs.gnu.org
> Date: Wed, 13 May 2015 23:52:35 -0400
> 
> > discovers that it's unibyte.  At least, that's how I did it: by starting
> > xterm in an Ubuntu 15.04 environment where LC_ALL=en_US.iso885915, and then
> 
> Oh, but that's the easy case.  The real test is when the environment
> uses utf-8 but the terminal's font fails to include the glyphs.

IOW, the terminal supports UTF-8, but the font it uses doesn't have
these characters covered?

Is that something that is likely to happen in real life?  I was under
the impression that the General Punctuation block was well covered in
UTF-8 locales in general, and in fonts used for the default text when
UTF-8 encoding is supported, in particular.  But if that's not the
case, we could provide a user option to forcibly use the display table
with ASCII glyphs for these characters, even if char-displayable-p
says they can be displayed.