This allows an easy way to toggle seeing all glyphless characters in
the current buffer.
-+++
+---
*** The extra slot of 'glyphless-char-display' can now have cons values.
The extra slot of the 'glyphless-char-display' char-table can now have
values that are cons cells, specifying separate values for text-mode
and GUI terminals.
----
++++
*** "Replacement character" feature for undisplayable characters on TTYs.
The 'acronym' method of displaying glyphless characters on text-mode
frames treats single-character acronyms specially: they are displayed