Add .maintscript file, clean up maintscripts

- Add a .maintscript file and Depend: on debhelper 8.1.0 for that
  support.
- Punt the postrm and preinst, which only existed for
  maintscript-helper code
- Remove the maintscript-helper invocations from the postinst and
  prerm
Release 1.42

Release version 1.42