Use data_files instead of package_data with py2exe.

Distutils' package_data argument doesn't work with py2exe.
On the other hand, we don't need the data_files argument
(which we're only using under Linux for stuff like our .desktop file and
man page that go to system directories), so we'll just use it instead
of package_data.
install.nsi: Add line to Delete "$INSTDIR\_ctypes.pyd".
Add an "nsis" target to setup.py to run NSIS.

That way, NSIS' location is determined by its registry key,
which makes nsis.bat obsolete.