febootstrap/helper: Major change to kernel module handling code

febootstrap/helper: Major change to kernel module handling code

The previous implementation had two problems: (I) Not all needed
kernel modules were copied to the initrd. (II) If a kernel module
depended on more than one other module, only the first dependency was
considered for the load order. Using 2.6.39-1-amd64 (Debian/unstable)
ext2.ko was not loaded and therefore the root FS could not be mounted.

The use of tsort(1) has been replaced with a set of functions that
build a DAG in memory and use that to calculate the list of modules to
be copied into the initrd and the order in which to load them.