(svn r3367) Unify the 4 distinct CMD_CHANGE_{AIRCRAFT,ROADVEH,SHIP,TRAIN}_SERVICE_INT commands into one CMD_CHANGE_SERVICE_INT command.

As side effect this is a
-Fix: The default AI tried to change the service intervals of vehicles via the CMD_CHANGE_TRAIN_SERVICE_INT command - regardless of the type of the vehicle - which of course failed for non-trains

command.h

@@ -58,7 +58,7 @@ enum {
 	CMD_DELETE_ORDER = 44,
 	CMD_INSERT_ORDER = 45,
 
-	CMD_CHANGE_TRAIN_SERVICE_INT = 46,
+	CMD_CHANGE_SERVICE_INT = 46,
 
 	CMD_BUILD_INDUSTRY = 47,
 
@@ -81,7 +81,6 @@ enum {
 	CMD_START_STOP_AIRCRAFT = 60,
 	CMD_BUILD_AIRCRAFT = 61,
 	CMD_SEND_AIRCRAFT_TO_HANGAR = 62,
-	CMD_CHANGE_AIRCRAFT_SERVICE_INT = 63,
 	CMD_REFIT_AIRCRAFT = 64,
 
 	CMD_PLACE_SIGN = 65,
@@ -92,7 +91,6 @@ enum {
 	CMD_SELL_ROAD_VEH = 69,
 	CMD_SEND_ROADVEH_TO_DEPOT = 70,
 	CMD_TURN_ROADVEH = 71,
-	CMD_CHANGE_ROADVEH_SERVICE_INT = 72,
 
 	CMD_PAUSE = 73,
 
@@ -113,7 +111,6 @@ enum {
 	CMD_SELL_SHIP = 87,
 	CMD_BUILD_SHIP = 88,
 	CMD_SEND_SHIP_TO_DEPOT = 89,
-	CMD_CHANGE_SHIP_SERVICE_INT = 90,
 	CMD_REFIT_SHIP = 91,
 
 	CMD_CLONE_ORDER = 99,