nipapd: change shebang to /usr/bin/env python

The old shebang does not work well with e.g. virtualenvs.
nipap-passwd: change shebang to /usr/bin/env python
xmlbench.py: remove duplicated shebang
xml-test.py: change shebang to /usr/bin/env python
Merge pull request #583 from booo/python3

Change shebang to /usr/bin/env python