Modified initial property parsing order to be more intuitive. It now

Modified initial property parsing order to be more intuitive.  It now
works like this:

1. Parse preferences.xml, which will usually specify a default
aircraft using the /sim/aircraft property.

2. If /sim/aircraft is specified and not empty, parse the properties
in $FG_ROOT/Aircraft/{/sim/aircraft}-set.xml to set up a default
aircraft.

3. During command-line parsing, whenever the --aircraft option appears
set /sim/aircraft appropriately and parse the properties in
$FG_ROOT/Aircraft/{/sim/aircraft}-set.xml.  Any --config, --prop, or
--aircraft options afterwards will take precedence.