Fix the "blue ribbon" #2911

This patch tries to fix the ribbon that gets drawn when NPC ships are docked.

It resets the trails of docking/docked NPC ships and clears the frame to nullptr.
In the update loop this is checked for and reset to the current bodies frame.

@radius75 - I think this will need testing with some NPCs to see what happens to their trails after they undock from a space station before it can be merged.