Cleanup pkla code and support logind

In Trusty, the policies for suspend and hibernate have moved from
upower to logind.  So we add groups to the pkla files to disable
those logind actions.  Note that logind differentiates between
actions for seats that support single sessions vs multi sessions,
and all Unity sessions are multi-user sessions, by definition.
(Because Unity.)

Also, merge the pkla files to a single file in /var/lib/polkit-1,
with a hierarchical name, and add maintscript-helper code in the
maintscripts to punt the old etc conffiles on upgrade.
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