Reduce memory v2

Changes in v2:

* Add a second patch that adds the -F option to _not_ salvage files even if -a is given. Turns out that salvaging files allocates another `total_cluster_bytes*sizeof(uint32_t)`, so I had to bypass that.
* Fix whitespace (my editor isn't able to do the crazy tab+spaces combination).
* Unmap the mmapped `fs->fat` when read_fat is called a second time.