Drop support for gdm and gconf2; wrap Depends

debathena-cluster is now only supported on releases that support
lightdm, so drop all our gdm code, along with all gconf2 support
(you must support gsettings)

Wrap the Depends field, and depend on lightdm-config now
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