The quotes form is normally only used for headers with path relative
to the including file's path, though the standard doesn't strictly
mandate this. This is consistent with the rest of sg/fg, it makes the
code's intent clearer and helps to find headers. (And it's a few
milliseconds faster, too.)
This makes taxiways smaller (important since at present there are so many).
Restructure the apt.dat parsing code to use a helper class instead of one long
function, and to do less work when parsing the file.
Some of these ideas come from Yon Uriarte's patches - thanks Yon.