logind: return "no" if sleep operation is disabled

logind: return "no" if sleep operation is disabled

According to org.freedesktop.login1:

> If "na" is returned, the operation is not available because
> hardware, kernel, or drivers do not support it. If "yes" is
> returned, the operation is supported and the user may execute
> the operation without further authentication. If "no" is returned,
> the operation is available but the user is not allowed to execute
> the operation.

Therefore, we should return "no" if sleep is explicitly disabled,
otherwise we return "na".