Add a bidi-control target for `glyphless-char-display-control'

Add a bidi-control target for `glyphless-char-display-control'

* doc/lispref/display.texi (Glyphless Chars): Document bidi-control.

* lisp/international/characters.el (char-acronym-table): Add the
ISOLATE characters.
(update-glyphless-char-display): Implement bidi-control, and mark
all Cf characters we don't know about as UNK.
(glyphless--bidi-control-characters): New variable.
(glyphless-char-display-control): Add a new type -- bidi-control.

* lisp/textmodes/glyphless-mode.el (glyphless-mode-types): Add a
new type -- bidi-control.