use the correct dbo

There are several cases of table classes which internally use instances of themselves or other table classes but get these instances without specifying a dbo. In normal Joomla operation, this will never be a problem but, when you try to do something funny, like manipulate tables on some other db, you may have problems. 

FOFTable may have the same issue but I don't understand the purpose of that class well enough to touch it right now.