Use fixed size for directory entries records, instead of semantic type
in the struct which is used to access on-disk records.
Change-Id: If8816d367eaa7070ccb17202ffc9047b5aee5d3e
. move mfs-specific struct, constants to mfs/, so
mfs-specific, on-disk format structs and consts are
fully isolated from generic structs and functions
. removes de and readfs utils
