Pilotfile should preserve unknown data

The pilotfile code should be able to detect unknown data and preserve it in subsequent saves, especially when the file version is newer than what FSO is expecting.

This is to allow for backward compatibility of newer pilotfiles within older FSO versions.

Enforcing the "Only add, never remove" rule for the player .JSON will ensure that newer PLR versions will be readable by old FSO versions, since the new PLR versions will just have new data that the old versions don't have to work with. However, if we want the ability to remove old data then the pilotfile::load_player() and handlers will have to be updated to gracefully recover.