Threading update to bring the sound process in to line with all the other processes (mostly). This might fix bug 1362778.
Remove all traces of ThreadAbortException by providing a controlled shutdown of all threads - including the loaded when it is otherwise busy.
Removes the sound process lock (SoundSources) in favour of an interlocked compare-exchange.
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@2616 a9f3cfff-6c41-7446-b0ca-052bd761e6eb