[screenlocker] Grab XServer while establishing the grab
Review Request #122528 - Created Feb. 11, 2015 and submitted
By grabbing the XServer we can ensure that no other client is sending X events. This eliminates a possible timing attack in the time frame between grabbing keyboard and pointer. At the same time the waiting and try again becomes useless as the XServer is grabbed and no other client could release the hold input device grab.