Fast aircraft velocities wrongly displayed as `0kts`

[ndim]

Fast aircraft can fly so fast that mpmap disregards the speed and just displays 0kts. There are two issues with that:

1. Having a kind of plausibility filter does make sense, but has become difficult to do and makes less sense with the number of supersonic FlightGear aircraft available these days.

2. Displaying 0kts makes no sense for aircraft filtered out for moving too quickly. A display string which cannot be confused with a stationary aircraft such as e.g. ?kts or just leaving an empty string would make much more sense IMHO.

This applies to both the old mpmap and the new v3 mpmap.