xinput: req GrabDeviceKey.modifier_device: altenum

xinput: req GrabDeviceKey.modifier_device: altenum

add altenum ModifierDevice to field modifier_device for value UseXKeyboard

V2: patch revised according to info from Ran Benita about NULL and UseXKeyboard

spec and code:
http://cgit.freedesktop.org/xorg/lib/libXi/tree/specs/encoding.xml#n1147
http://cgit.freedesktop.org/xorg/proto/inputproto/tree/XI.h#n187
http://cgit.freedesktop.org/xorg/xserver/tree/Xi/grabdevk.c#n116

the following spec uses "NULL" instead of "UseXKeyboard":
http://cgit.freedesktop.org/xorg/proto/inputproto/tree/specs/XIproto.txt#n1213

The reason for NULL vs UseXKeyboard is: in the libXi function XUngrabDeviceKey,
the modifier_device arg is an XDevice pointer, and NULL means to use the
core X keyboard. But on the wire it is an integer, with 255 as a special
value.