According to systemd logind's documentation, the CanSuspend() method
"returns 'na' in case "the operation is not available because hardware,
kernel or drivers do not support it", while "'no' is returned if the
operation is available but the user is not allowed to execute it".
See http://www.freedesktop.org/wiki/Software/systemd/logind
Thus, we need to return true here when the reply for the CanSuspend
method is neither 'no' nor 'na', or we would providing false positives
in cases where suspension is simply unsupported.
https://bugzilla.gnome.org/show_bug.cgi?id=748338
be78f0f36a