libmfu: Develop cache file format using variable length fields

The current cache file format uses fixed length records. It encodes the full path to each file, and to produce a fixed length field, it uses the longest path name of any file. So when writing the cache file for a large number of files in which there may be a really long file name, the output file size is inflated.

To conserve space, let's store this using variable length records. To support that efficiently, we'll either need a table in the header describing the offset of each record, or we'll need to add some end-of-record marker between records.

Start by locating the libmfu cache file implementation and reading how its fixed-length records and full paths are written and read. Decide whether a header offset table or end-of-record markers fit the existing format; done means variable-length records conserve space while remaining readable by the cache logic.