This is too destructive for many established things, so revert it.
Instead, make the FDM shell save /fdm state on postinit(), and
restore it on re-init (which reposition invokes).
As part of this, kill off the evil global FDM state, and avoid us
copying FGInterfaces (which is bad since it contains TiedProperties
and an FGGroundCache, neither of which are especially keen on being
copied). Will probably disable copy/assignment on TiedProperties in
a future commit.
