Another shot at multi-threading

[manacker@...]

Just for fun, I combined Lua 4.0 and the LGPL'd ZThread-1.2.1 portable
thread package.  It's really early code, but I can now start a script
in another thread and kill it at will.  Each lua_State lives in it's
own thread, so it's really more of a Lua-centric multi-tasking system
than true threading (as far as the Lua scripts are concerned).  Next
up are the IPC mechanisms - semaphores, mutexes, and data distribution
systems.  Ultimately, a script running in a thread will have an easy
way to send and receive messages from the other threads running in
that context.

I'll probably stick the codebase over on sourceforge, since I've
already got a space there.  It'll do until we get real threads
integrated into the core of Lua.