This adds sleep-inhibitor as a dependency when elogind is present, and
installs a conf for it with some plugins enabled that are beneficial to
devices running pmOS. The elogind suspend hook is dropped.
This resolves issues where suspend is aborted in the hook *after*
PrepareForSleep is sent over dbus (which might cause some things to
disable radios/modems, etc for suspend).
The 'period' in the conf is in minutes, a value of 0.1 is 6 seconds,
which seems reasonable for releasing the lock when conditions for
inhibiting no longer apply, without spinning the CPU too frequently.
