bugfix on collect_shmem_active_info()

It incorrectly calculated a series of contenious blocks, because
of padding size.
correct one is:
int     nshift = find_least_pot(offsetof(shmem_body, data[0]) +
                                            block->blocksz +
                                            sizeof(cl_uint));
code to launch gpusort kernel
prototype adjustment of run_gpusort_multi
fixbug: fixed-length data type but attbyval=false didn't cached correctly