Keep assisted ferry routes alive

One of the coolest but majorly underused features are ferry routes.

- You can set land factory waypoint to the route to send all units along
- you can set air factories waypoints to the route so new transports assist the ferry route

However: as soon as the initial transport gets shutdown, the ferry is removed completely. This makes investing time into complex routes a lost cause.

Suggestions: either
a) if the leading transport dies, give ownership of the ferry route to a random assisting transport
b) keep the ferry route even if all transports die so you can reuse it (you have to remove it manually)