xinput: req UngrabDeviceKey.modifier_device: altenum

add altenum ModifierDevice to field modifier_device for value UseXKeyboard

spec and code:
http://cgit.freedesktop.org/xorg/lib/libXi/tree/specs/encoding.xml#n1175
http://cgit.freedesktop.org/xorg/proto/inputproto/tree/XI.h#n187
http://cgit.freedesktop.org/xorg/xserver/tree/Xi/ungrdevk.c#n113

the following spec uses "NULL" instead of "UseXKeyboard":
http://cgit.freedesktop.org/xorg/proto/inputproto/tree/specs/XIproto.txt#n1295
Questions:
* is this spec wrong?
* or does "NULL" have the unusual value of 0xff in this context?