Update --showaircraft and --aircraft= options to recursively search the

Update --showaircraft and --aircraft= options to recursively search the
$FGROOT/data/Aircraft hierarchy.  There could be some long term performance
concerns if a person has a *huge* collection of aircraft or a really slow
file system, but I see zero performance blip here from recursing the default
CVS tree.  We should also allow the user to specify the whole path to the
-set.xml file if they don't want to recurse ... this way we could eventually
come up with an aircraft selection dialog box on the front end so the user
could manually walk the tree to the desired aircraft.  There also the system
wouldn't have to search for the aircraft.