powerdown: Poke port 28 manually rather than talk to firmware

The built-in powerdown function will probably be at a different
address on different firmware versions, and it doesn't work properly
with our hijump/lojump shenanigans anyway.

This makes the Power button work again, but I still need to figure
out what else to change before shutting down so the next boot
doesn't go to the "Reset System Data" screen first.

lib/crt0.s

···
 _exit::
 	call	_reboot
 
-_powerdown_mode::
-	call	#0x0a6b			; firmware powerdownmode function
+_powerdown::
+	; TODO: figure out what else needs to be set before shutting down to
+	; prevent it going to the "Reset System Data" screen on next startup
+	ld	hl, (#p28shadow)
+	ld	a, (hl)
+	set	1, a			; 74c74 pin4
+	set	0, a			; modem reset
+	ld	(hl), a
+	di
+	out	(#0x28), a
+	halt
+	ret
 
 _reboot::
 	jp	0x0000

lib/mailstation.h

···
 
 /* crt0.s */
 extern void exit(void);
-extern void powerdown_mode(void);
+extern void powerdown(void);
 extern void new_mail(unsigned char on);
 extern void reboot(void);
 extern void delay(unsigned int millis);

main.c

···
 
 unsigned char lastkey;
 
-void process_keyboard(void);
+int process_keyboard(void);
 void process_input(unsigned char b);
 
 int
···
 	printf("Hello, World\n");
 
 	for (;;) {
-		process_keyboard();
+		if (process_keyboard() == KEY_POWER)
+			break;
 	}
+
+	return 0;
 }
 
-void
+int
 process_keyboard(void)
 {
 	unsigned char b;
···
 		lastkey = b;
 
 	if (b == 0)
-		return;
+		return 0;
 
 	switch (b) {
 	case KEY_POWER:
-#if 0
-		/* XXX: this triggers erroneously */
-		powerdown_mode();
-#endif
+		delay(100);
+		powerdown();
 		break;
 	case KEY_F1:
 		break;
···
 	default:
 		putchar(b);
 	}
+
+	return b;
 }