Now with cancel button and an actual progress bar. For simplicity, we do
two passes on the progress bar, one for loading the NAND into memory and
one for extracting it. The user directory is likely on an SSD, making
the extraction pass invisibly fast.

There is no need to constantly reset the key for every file entry.

Create a struct describing the superblock layout and use it directly
without needing to specify offsets and such.

It also simplifies the code flow, as it no longer goes backwards
through the filesystem chain.