Physical drives error counters datapoint type change

Hello, Egor.

I've changed gauges to derives for several counters. The reason I did this is constant alerting from zenoss in case an hdd has non zero error counter (imagine, it has 5 lifetime errors. Zenoss would alert about it as a critical condition).
MIB tells that these values are given for last "phyDrvRefHours" period. But I couldn't find nor its value, nor oid.
I have also added some "rrdmin" values for every derive I've found, because these values declared as counters in MIB.
Please review my amendments, I'm not sure all of them are correct and appropriate.

Additionally, I have a question about controller status threshold: would you accept a pull request with severity gradation for controller status? At the moment, you have only "status" threshold. And I would like to see there several min-max threshold with a separate event for each condition defined in MIB.