#553: MP-model-loading related segfault

#553: MP-model-loading related segfault
MP aircraft are loaded by a separate OSG thread (introduced after FG2.4.0).
The OSG thread also calls the "modelLoaded" callback. However, we mustn't
allow the OSG thread to call FGNasalModelData::modelLoaded directly:
FGNasalModelData isn't thread-safe. There are obvious issues (_callCount++;),
tricky issues like calling the Nasal _parser_ or creating hashes and
modifying the global Nasal namespace. It doesn't use locks to protect
against another thread executing a Nasal context or running garbage
collection. It also executes Nasal code itself (the model's "load" hook),
which we also cannot allow in a separate thread...
This patch returns all Nasal parts of MP-aircraft loading (parsing,
module creation, execution) to the main thread, while keeping the
multi-threaded OSG part (loading of MP-aircraft model files itself).
The same issue exists with scenery models (see other commit).

To summarize with 2 words: It s*cks... ;-)