Better support for extra data for SRU codecs and GFS

Better support for extra data for SRU codecs and GFS

The Z_SRW_PDU structure has two new members extraResponseData_{buf,len}
for extra data response buffer and length. For the GFS, both request
and response data (XML) is carried in extra_args and extra_response_data
for the search handler. Patch by Ko van der Sloot.