engine: the Japanese keyboard's own keys; keypad arrows move through candidates

半角/全角, ひらがな/カタカナ, 変換, and 無変換 committed the reading and did
nothing, like any function key.  They now stand for what a Japanese
keyboard means by them: toggling Japanese and English, switching kana
modes, converting (or turning Japanese on), and turning it off; the
Korean keys became a switch of the same shape.  The keypad's arrows
and page keys, with NumLock off, fold onto the plain ones as its Enter
and Tab already did.

tests/ipc_test.c

@@ -55,6 +55,7 @@ ipc_masks_modifiers(struct ct *t)
 	CT_EQ_UINT(t, Kret, ipckeysym(0xff0d, '\r'));
 	CT_EQ_UINT(t, Kret, ipckeysym(0xff8d, '\r'));	/* KP_Enter */
 	CT_EQ_UINT(t, Ktab, ipckeysym(0xfe20, 0));	/* ISO_Left_Tab */
+	CT_EQ_UINT(t, Kdown, ipckeysym(0xff99, 0));	/* KP_Down */
 	CT_EQ_UINT(t, Kback, ipckeysym(0xff08, 8));
 	CT_EQ_UINT(t, Kspec + 0xff, ipckeysym(0xffff, 0x7f));	/* Delete */
 	CT_EQ_UINT(t, 0x1f642, ipckeysym(0x101f642, 0x1f642));