Further work on FS init

* File system superblock stores info in lowercase (because it makes a
  few things easier - for now)
* During mount, the file system is “sanity” checked - magic number,
  block count, and algorithm choices

Speaking of which: any algorithms supported by mcrypt and mhash should
now work. Although you will (currently) have to modify mkfs , as until
we've completed some more testing we're a little reluctant to fully make
that available.