Different Scaling of Port Bits Graphs

This patch adds some options to change the scaling of port bits graphs
It allows to use the current autoscaling of rrdgraph as default but adds some other.
You can use ifSpeed as scale and also force scales like 100Gbit, 10gbit, 1gbit, 100mbit

Note:
i replaced my bit graphs with mrtg style graphs but this code is not affected