PC-98: Implement the kanji/graph mode flag

When outputting characters via INT 29h, NEC's IO.SYS actually only
decodes Shift-JIS, and writes full-width JIS characters to text RAM,
if the byte at 0060:008A is 1. Setting it to 0 switches to "graph
mode", where Shift-JIS decoding is disabled, and the codepoints between
0x81-0x9F and 0xE0-0xFF are also written to text RAM directly. They're
then rendered as the half-width glyphs at those corresponding locations
in font ROM.

On original NEC PC-98 DOS, these modes can be toggled via Ctrl+F4, or
the escape sequences `ESC )3` (char mode) and `ESC )0` (kanji mode).

src/dos/dev_con.h

@@ -360,7 +360,7 @@ private:
     }
 
 	static void Real_INT10_TeletypeOutput(uint8_t xChar,uint8_t xAttr) {
-        if (IS_PC98_ARCH) {
+        if (IS_PC98_ARCH && (real_readb(0x60, 0x8A) == 1)) {
             if (con_sjis.take(xChar)) {
                 BIOS_NCOLS;
                 uint8_t page=real_readb(BIOSMEM_SEG,BIOSMEM_CURRENT_PAGE);
@@ -536,7 +536,7 @@ private:
 			break;
 		default:
 			//* Draw the actual Character
-            if (IS_PC98_ARCH) {
+            if (IS_PC98_ARCH && (real_readb(0x60, 0x8A) == 1)) {
                 if (con_sjis.take(chr)) {
                     BIOS_NCOLS;
                     unsigned char cw = con_sjis.doublewide ? 2 : 1;

src/ints/int10.cpp

@@ -1219,6 +1219,7 @@ void INT10_Startup(Section *sec) {
          * Compare that to IBM PC platform, where segment fills only 0x40:0x00 to 0x50:0x00 inclusive and extra state is held in the "Extended BIOS Data Area".
          */
 
+        real_writeb(0x60,0x8A,1); /* kanji/graph mode */
         real_writeb(0x60,0x8C,' '); /* function row mode 2 indicator */
 
         /* number of text rows on the screen.