[Ovmsdev] units

I noticed that changing the config option for preferred units caused the display of distance on the status page to show the wrong units. AsUnitString() correctly converted the value to the desired units, but still appended the native unit name. While looking at that code, I also noticed that the metric unit abbreviations don't conform to standard SI notation: - kilometer should be 'km' (not 'Km' because capital-K means Kelvin). - kilometers per hour is better as 'km/h' (not 'Kph' because p means pico-). Here is a pull request to fix both: https://github.com/openvehicles/Open-Vehicle-Monitoring-System-3/pull/152