ASUS xtion: use serial number

Hi,

currently the ASUS xtion cameras have an empty/constant "device_id", obviously because they don't advertise a serial number the way be Microsoft Kinect does.

However, there is another serial number available in the prime sense driver, and this one
is also present on ASUS cameras.

This patch is a proof of concept that reads out this alternative prime sense serial number in case the main one is empty/constant.

I've tested this patch with
- 2 Kinects
-  2 ASUS
- 1 Kinect, 1 Asus
without observing any crashes or other problems.

HOWEVER: I'm far from being an expert regarding the openni driver and these patches may very well introduce bugs, race conditions or worse. Especially the fact that not applying the 3rd patch (that removes some inconspicuous looking lines) results in segfaults makes me somewhat sceptical.

So this is less a "take these patches at face-value" and more of a "please apply something similar to the stuff I sent you" pull request. ;)

Cheers
Dariush