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 of the SFrame header may
need to be version aware.

flip_header () now takes the SFrame version as argument and also returns
SFRAME_ERR in case of error.  Currently the SFrame version as argument
remains unused.

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 flip_header.
        (sframe_encoder_write): Likewise.