RRDModel: change data source type to DERIVE
RRDModel: change data source type to DERIVE

I misunderstood the RRDtool docs; DERIVE is the best DST for the traffic
counters, not COUNTER, because COUNTER interprets a reset to zero as
overflow, resulting in huge spikes unless given an arbitrary maximum.

welshjf/bitnomon,welshjf/bitnomon