Bug#500084: difficult to diagnose failure when user is not in powerdev

[Rolf]

If a user is not in the powerdev group, the suspend/hybernate fails,
with a reference to the quick website, which is of no help.
There is a reference in the readme.Debian add the user to the powerdev
group.

In the source, after a suspend failure, only a failure is reported, the
original cause is lost.

Probably best to check upfront if the user is allowed to suspend in hal,
and disable suspend option. This requires an patch for
hal_gpower_can_suspend function in libhal-glib/libhal-gpower.c (see
TODO: Change to can_suspend when rely on newer HAL)

my two cent
Rolf