[PATCH] sphinxSE GROUP BY on multiple attributes

It is possible to group by on multiple attribute values with the native API and SphinxQL but not with the Storage Engine.

The patch implements a new mode ("multi:") for the groupby function SPH_GROUPBY_MULTIPLE.

After the patch it is possible to group by on multiple attribute values with this query:

SELECT _sph_count, attribute1, attribute2 FROM sphinx_index_table WHERE query = ';groupby=multi:attribute1, attribute2';