Added test that jailed code can't fork.

Python should have -E, not sure of a clean way to do it, but this at least only applies it to python.

We would fail if a global was defined with a non-jsonable value inside a jsonable one.  Now we don't/

Files are properly copied in both implementations of safe_exec, and a new python_path argument adds to the python path.

Implement safe_exec on top of jailpy (old unsafe safe_exec is still here); Remove some crazy stuff from the context; always pass globals and locals, locals are the things that can be changed.