add pg_strom.debug_force_gpupreagg option, to enforce GpuPreAgg for debugging
add message if GpuPreAgg re-checks chunk by CPU.

Example)

postgres=# select key, avg(x) from tx group by key;
NOTICE:  GpuPreAgg: 1 chunks were re-checked by CPU
 key |                                              avg
-----+------------------------------------------------------------------------------------------------
 ddd |  38.7295488733798420
 bbb |  49.3370704539120370
 ccc |  44.2985584773123300
 aaa | 9090909090909090909090909090909090909091818181818181818181827272727272727272751.31100197322666
(4 rows)