Linkbacks with sub-classes

Nodes defined as a subclass of another class will attempt to create a linkback for each parent class if the target exists. If no appropriate targets are found, a linkback to the `.default' file is created.

For example, if a node in the `compute.gige' class has a `CLASS' linkback defined for /etc/fstab, oneSIS will attempt to linkback to /etc/fstab.compute.gige. If that target doesn't exist, it will attempt to linkback to /etc/fstab.compute, then finally /etc/fstab.default.