The changed thread code may not wish to save the old context under
certain circumstances but thread-unix.c assumed it would, cached it
and used it unconditionally.
Also, prevent it from leaking away all the jump buffers (old problem).
Creating and removing threads would eventually run it out of buffers
and then it would crash after that. Plugins, like Pictureflow, which
have worker threads could only be started a few times. Implement a
simple O(1) allocator that will reuse them and stays self-contained
to its own types (as it appears the original author intended).
Change-Id: Icf65413c086b346fb79bf827102b725269e2812c
a9713d89e7