[tvgqvnnt] Filesystem within disjoint containers

Create a filesystem backed by files in other filesystems.  However, unlike the simple way of allocating a single file and creating and mounting loopback a filesystem within it, let this be backed by multiple files, likely files in several different filesystems mounted within a single directory structure.

The motivation was starting with a bunch of partitions with a little bit of free space in each, wanting to combine all the free space to create the illusion of a single directory with lots of free space.

In addition, let the size of the inner filesystem not need to be chosen a constant at the time of creation.  Instead, the space available is simply the sum of the space available in the outer filesystems.