New option "display_borders" to display 0 pixel table borders.

cf. http://d.hatena.ne.jp/rubikitch/20101120

fm.h

@@ -1097,6 +1097,7 @@ global char UseAltEntity init(FALSE);
 #define GRAPHIC_CHAR_DEC 1
 #define GRAPHIC_CHAR_CHARSET 0
 global char UseGraphicChar init(GRAPHIC_CHAR_CHARSET);
+global char DisplayBorders init(FALSE);
 extern char *graph_symbol[];
 extern char *graph2_symbol[];
 extern int symbol_width;