fgInitAircraft() loads the aircraft -set.xml file, which calls
ResourceManager::findPath() for each 'include=...' directive. In order
to prevent such includes from triggering the new warning printed by
ResourceManager::findPath(), call fgInitAllowedPaths() after the
aircraft paths have been updated and before the -set.xml file is loaded.
Thanks to Alan Teeder for reporting the problem.
We can now probably remove the call to fgInitAllowedPaths() after
processOptions() in fgStartNewReset() (I believe the newly-added call
comes “soon enough”). Not doing so in this commit, though: let's fix
problems before optimizing.