libsframe: make flip_header version aware

libsframe: make flip_header version aware

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

flip_header now takes the SFrame version as argument.  Currently unused,
this argument will be used later when we introduce SFrame V3.

SFrame encoder, at the momemnt, writes the SFrame data in the most recent
format version by default.

libsframe/
        * sframe.c (flip_header): Make version aware.
        (sframe_decode): Adjust usage of flup_header.
        (sframe_encoder_write): Likewise.