Fix bitmap_sfu() to shift by LG_BITMAP_GROUP_NBITS rather than hard-coded 6 when using linear (non-USE_TREE) bitmap search. In practice this affects only 64-bit systems for which sizeof(long) is not 8 (i.e. Windows), since USE_TREE is defined for 32-bit systems. This regression was caused by b8823ab02607d6f03febd32ac504bb6188c54047 (Use linear scan for small bitmaps). This resolves #368.