When I run the test code, ^C once does nothing; ^C twice takes
me back to the Unix prompt.
In lua.c, laction is called and sets a hook in the main thread to
interrupt Lua cleanly next time it gets a chance, but it never does
because control is with the child thread.
The second ^C kills the program because laction sets the SIGINT handler
to the default action, which is to terminate the process.
The question seems to be whether debug hooks are set per Lua thread or
per Lua state. I think that in previous versions this was different from
what it is now, and so other issues arose.
