Clean up naming of the 'point on runway' helpers, to get rid of the confusing

Clean up naming of the 'point on runway' helpers, to get rid of the confusing
notion of a 'displacedThreshold'. Now there's just a real threshold,
displaced or otherwise, and people who care about the paved area can use
'begin' and 'end'. Thanks to John Denker for pointing out the confusion this
leads to. Using 'end' also gets rid of the 'reverseThreshold' name, which was
clearly a bad choice of mine.