AbstractAirspace: pass rvalue tstring reference

Reduce runtime overhead.
AbstractAirspace: don't reset radio in SetProperties()

None of the two callers expect that, and this just adds useless
overhead.
AirspaceParser: move empty check to AddPolygon()
AirspaceParser: discard polygons with less than 3 points

Change AirspacePolygon runtime checks to assertions.
AirspaceParser: use vector::front(), ::back()