py2exe currently has an undocument per-target
unbuffered option, but that option does not actually
disable buffering in the target executable.
This patch changes from using PYTHONUNBUFFERED
(which is not looked at by Py_Initialize but in Python
itself is handled in frozenmain.c) to explicitly disabling
buffering itself in start.c. This is also the approach
taken in Gordon McMillan's installer package.