Quota usage: map soft/hard quotas to warning/critical thresholds

Hi,

this change maintains the warning/critical thresholds on the quota usage using configured soft/hard quotas.

I am using this change in production since 8 months, with no problems. I also managed to got this patch included in [OpenSUSE's server:monitoring package](https://build.opensuse.org/package/show/server:monitoring/munin).