libsframe: make flip_fde version aware

Future versions of the format may have a different representation of an
SFrame FDE.  As the format evolves, endian flipping will need to be version
aware.

Refactor flip_fde a bit by carving out an internal sframe_decode_fde API
which can read information from an on-disk SFrame FDE.

libsframe/
        * sframe.c (flip_fde): Make version aware.
        (sframe_decode_fde): New internal definition.
        (flip_sframe): Use the new definitions.