cleanup_python and python_done_keybinding_interface called by it
now reset the PyObject *python_hooks, *keybindings variables back
to NULL when they release the references. Without this change,
dangling pointers left in those variables could cause problems
if the Python scripting module were deinitialized and reinitialized.
It looks like such reinitialization is not currently possible though,
because enhancement request 73 (plugins support) has not yet been
implemented.
Before this patch, init_python would crash trying to set up elinks.home
at the Python side. Now it uses None as the value in that case.
Also, init_python no longer adds "(null)" to $PYTHONPATH.
