Codechange: Template DoCommandP to automagically reflect the parameters of the command proc.

When finished, this will allow each command handler to take individually
different parameters, obliviating the need for bit-packing.

src/order_backup.cpp

@@ -171,7 +171,7 @@ CommandCost CmdClearOrderBackup(DoCommandFlag flags, TileIndex tile, uint32 p1,
 		/* If it's not a backup of us, ignore it. */
 		if (ob->user != user) continue;
 
-		DoCommandP(CMD_CLEAR_ORDER_BACKUP, 0, 0, user);
+		Command<CMD_CLEAR_ORDER_BACKUP>::Post(0, 0, user, {});
 		return;
 	}
 }