We may need more than one handler to deal with a property (e.g. one in the
driver, one in the DIX), so get the handlers into a linked list and call them
one-by-one. This is of course slightly less entertaining than the hilarious
WRAP/UNWRAP game we play in other parts of the server.
XIRegisterPropertyHandler/XIUnregisterPropertyHandler are the interface
drivers/the DIX should use to attach themselves to the device.
XIDeleteAllDeviceProperties destroys everything, including the handlers.
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