Stash the env PYTHONPATH during startup and restore it in main

Stash the env PYTHONPATH during startup and restore it in main

We have to set the PYTHONPATH from bup.c so that Py_Main will be able
to find bup.*, but we don't want that change to affect subprocesses,
so stash the original PYTHONPATH in bup_main.env_pythonpath, and
restore the original environment value once we've made it to the
python side (in bup.main).

Thanks to Johannes Berg for pointing out some issues with a previous
version of the changes.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>