commands.import.airspace: adopt xcsoar's airspace class parsing

This will make it easier to use xcsoar's airspace intrusion code.
model.airspace: use xcsoar for airspace infringement detection

In general this is twice as fast as the postgis approach, speeding
up to 5 times for flights through many airspaces.

As a side effect, xcsoar already knows how to handle flight levels
or altitudes above ground.
model.airspace: return uppercase airspace references

And fix the SI units in get_airspace_infringements
model.airspace: replace NOTAM and UNKNOWN airspace references

XCSoar doesn't know them. Replace with MSL for now...
mapserver: update airspaces
airspace.map: update airspace class names
model.airspace: check configured airspaces only
model.airspace: use QNH if available