system-monitor: Fix net speed
We use different formats for values above and below 10, to only include a fraction for the latter.
However we use the same factor to determine the format and to compute the number to format, with the result that values above 10 are off by a factor of 10.
Address this by adding a separate "unitFactor" value for computing the value.
Spotted by Yannick Daveluy.
Closes #494 (closed)