Prevent saving linked_to_menu as published and vice versa

[cmb69]

Currently, it is possible to load pagemanager with $cf['pagemanager']['pagedata_attribute']="linked_to_menu", to change that setting to $cf['pagemanager']['pagedata_attribute']="published" and to save pagemanager; that causes the published settings to be overwritten by the linked_to_menu settings; that works also the other way round, but neither appears to be a useful feature, but rather an oversight.