The get_sorted_session_displays() is broken in that it
doesn't produce a sorted list of displays.
The problem is the qsort comparison function which has 2 errors in 4 lines:-
1) The test is the wrong way round (i.e. arg1 < arg2 produces a +ve
result instead of -ve)
2) Subtracting two unsigned ints in C will never return < 0
The broken function has been masked by other display checks which mean
that it is only visible in a few situations:-
1) Starting two sessions very closely to each other may allocate the
same display to both sessions.
2) If /tmp is namespaced, the other display checks do not work, and
more than two sessions cannot be started.
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