Fixes#230
Incorporate github pull request #230, support for DECgraphics-style
line drawing in the curses interface. I've rewritten the
curses_convert_glyph() part so that it doesn't require C99 and
doesn't reinitialize its pair of arrays for every character written
to the map. The DECgraphics conversion is now a straight char for
char one, DEC line drawing code to ACS, without regard to what map
symbol is intended or what 'cursesgraphics' uses for that symbol.
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