vehicles: don't let drag stop vehicles in active cruise control

The current vehicle model has all the drag deceleration applied
at once at the start of each turn.  Vehicles that have more drag
deceleration than speed will stop, aligning their turn direction
with their facing.

This normally works fine, but some boats and amphibious vehicles
will have enough water drag deceleration to stop them every turn,
even if they have enough acceleration to return back to their
cruise control set speed.  Under those conditions, the vehicles
will move apparently normally across water, but cannot turn.

Give vehicles that are under cruise control with a non-zero
cruise control target velocity a nominal velocity if the drag
slowdown exceeds the vehicle's current speed, which seems to
resolve the problem.

src/vehicle.cpp

@@ -4245,7 +4245,11 @@ void vehicle::gain_moves()
         of_turn = 1 + of_turn_carry;
         const int vslowdown = slowdown( velocity );
         if( vslowdown > abs( velocity ) ) {
-            stop();
+            if( cruise_on && cruise_velocity ) {
+                velocity = velocity > 0 ? 1 : -1;
+            } else {
+                stop();
+            }
         } else if( velocity < 0 ) {
             velocity += vslowdown;
         } else {