alist used to display characters for which there's no glyphs in
the current codepage.
(IT-display-table-setup): New function, sets up the DOS terminal
for a given codepage.
(dos-codepage-setup): New function, sets up the MULE environment
for the current value of dos-codepage.
(top-level if): In the multibyte mode, turn on
unibyte-display-via-language-environment. In the unibyte mode,
set up the special syntax tables to map lower- to upper case and
back.
