Ehm, three unrelated changes.

- Add Py_FrozenFlag, intended to suppress error messages fron
getpath.c in frozen binaries.

- Add Py_GetPythonHome() and Py_SetPythonHome(), intended to allow
embedders to force a different PYTHONHOME.

- Add new interface PyErr_PrintEx(flag); same as PyErr_Print() but
flag determines whether sys.last_* are set or not.  PyErr_Print()
now simply calls PyErr_PrintEx(1).