src/ : Move to a variable length header buffer

Previously, the `psf->header` buffer was a fixed length specified by
`SF_HEADER_LEN` which was set to `12292`. This was problematic for
two reasons; this value was un-necessarily large for the majority
of files and too small for some others.

Now the size of the header buffer starts at 256 bytes and grows as
necessary up to a maximum of 100k.
save_header_info (SF_PRIVATE *psf)
{	snprintf (sf_parselog, sizeof (sf_parselog), "%s", psf->parselog.buf) ;
} /* save_header_info */

save_header_info (SF_PRIVATE *psf)
{	snprintf (sf_parselog, sizeof (sf_parselog), "%s", psf->parselog.buf) ;
} /* save_header_info */
