Renamed "Invalid" (airspace) to "custom"

More clang warnings cleanup

Change feature default values to something that will be identified as "not set". When a flight is created, features with missing values (not set) are not included. Note that all these are persistent, which may create a problem if something changes (different vehicle with large weight difference for instance). As the subsequent use of these are persistent, a question may not be asked because it's already set (the brief only asks for missing or problem values).