json_safe() ValueError for unpaired unicode surrogates

when passed unpaired unicode surrogates on Linux.

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.