xinput: rep ChangeKeyboardDevice.status: altenum to enum

xinput: rep ChangeKeyboardDevice.status: altenum to enum

Field "status" can only get values from enum GrabStatus
therefore make it an enum instead of altenum.

spec:
http://cgit.freedesktop.org/xorg/proto/inputproto/tree/specs/XIproto.txt#n1016

Add comment about valid values for field status.

Note:
An old spec lists value 2 for enum value "Frozen"
while enum GrabStatus has value 4 for Frozen.
The old spec is probably wrong.
Here is the URL of the old spec:
http://cgit.freedesktop.org/xorg/lib/libXi/tree/specs/encoding.xml#n1044