Blender Git Loki

Fix T40510, revert openmp thread count to how it was in
2.70 for non Apple systems.

Also refactored the code that restores the previous openmp thread count.
The logic here was weird, mostly due to all the commit madness with
Apple openmp support. The restored thread count though should not depend
on the on/off state of threaded sculpting (since it has to do with
systems other than sculpting only). For OSX threads are restored to the
system thread count but Jens should recheck here.