FGPositioned clean-ups - apply some desirable changes (such as making members

FGPositioned clean-ups - apply some desirable changes (such as making members
const) which were previously tricky but now easy. Make it possible not to
index certain types (used for taxiways) and exclude anonymous items from
the name index. Related to this, clean up FGRunway further - remove some public
members, and fix a dumb bug of mine, where we create reciprocal entries for
taxiways.

This should make startup (slightly) quicker, and shrinks FGRunway somewhat.