[Freeciv] FerryRoute addition

[banjo <banjo@xxxxxxxxxx>]

Hey

Here's an idea from Colonization, the idea of setting up FerryBoat
routes.

You can tell a ferry to ferry units between some points, this gets
added to the pathfinding algorithm as a traversable path.

The advantage being in lessening micromanagement.  If im attacking an
overseas area, then i can set up some FerryRoutes and then just 'goto'
units to the front-line, they'll travel to the pick-up point, wait, get
ferried to their drop off point and continue along.

As a server provided service, this will aid the AI players as well.

The ferry part of the route could be shown on the client as a dotted line.

Related to this would be the option of assigning units to guard
(or 'Convoy') a ferryboat. Both travel at the slower units speed.
This would be handy as harassing shipping routes would become more
common under ferrying.

-billy