(svn r15541) -Revert (r15399): 'v->cargo_type' is also used in other places, which cannot accept CT_INVALID.

-Codechange: Add Engine::GetDefaultCargoType() and Engine::CanCarryCargo() and use them.
-Fix [FS#2617]: When articulated parts have no available default cargo, use the cargo type of the first part for livery selection.
-Change: To decide whether a vehicle is refittable do not test its current capacity for being zero, but always use the 'capacity property'.
  Note: The property is used unmodifed without calling CB 15/36. By setting it to a non-zero value and returning zero in the callback vehicles can be refitted to/from zero capacity for e.g. livery effects.
  Note: It is intentional that you cannot control refittability by CB 36.

bin/ai/regression/regression.txt

@@ -1085,7 +1085,7 @@
   Engine 0
     IsValidEngine():    true
     GetName():          Kirby Paul Tank (Steam)
-    GetCargoType():     0
+    GetCargoType():     255
     CanRefitCargo():    false
     GetCapacity():      -1
     GetReliability():   75
@@ -1229,7 +1229,7 @@
   Engine 8
     IsValidEngine():    true
     GetName():          Chaney 'Jubilee' (Steam)
-    GetCargoType():     0
+    GetCargoType():     255
     CanRefitCargo():    false
     GetCapacity():      -1
     GetReliability():   79
@@ -1247,7 +1247,7 @@
   Engine 9
     IsValidEngine():    true
     GetName():          Ginzu 'A4' (Steam)
-    GetCargoType():     0
+    GetCargoType():     255
     CanRefitCargo():    false
     GetCapacity():      -1
     GetReliability():   83