Fix kbdmux(4) issue with backslash/underscore key not working on

Japanese 106/109 keyboard.

PR:		kern/112214, kern/99090
Submitted by:	TOMITA Yoshinori, TAKAHASHI Yoshihiro
Approved by:	re (hrs)
MFC after:	3 days

sys/dev/kbdmux/kbdmux.c

@@ -786,6 +786,9 @@ kbdmux_read_char(keyboard_t *kbd, int wait)
 		case 0x63:	/* wake key */
 			keycode = 0x6f;
 			break;
+		case 0x64:	/* [JP106USB] backslash, underscore */
+			keycode = 0x73;
+			break;
 		default:	/* ignore everything else */
 			goto next_code;
 		}