add knots to speed preferences

[plusOrInfinity]

I didn't add nautical mile to "preferred distance units" options, because it seems in GPSTest only "altitude" and "altitude (MSL)" use distance units. In aviation, altitude is measured in feet, and distance in nautical miles. Thus, supporting nautical miles in "altitude" and "altitude (MSL)" is not worth adding I think.

Fixes #390

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