Blender Git Loki

As mentioned in the pydrivers commit, I had to change the order in
exit_usiblender() to finalize Python before main library data was freed.
This solved a somewhat specific sigsegv with pydrivers, but as Ken
Hughes found out (thanks!) caused one with scripts that called Blender.Exit().

Now running scripts (G.main->script) are freed in BPY_end_python()
itself (so before the rest of the library data is freed), before
Py_Finalize(). Works fine in all my tests so far.

The file script.c should become obsolete with this change (I added a
comment about it there). If all is indeed fine, it will be removed
later.