Added secondary sorting (i.e., breaking ties when two documents have the same Q)

Added secondary sorting (i.e., breaking ties when two documents have the same Q)
on any of the custom attributes. Just define CUSTOM_SORTING in lib/custom.h.

I've also removed secondary sorting of result heap by site ID inside refs.c
-- according to my best knowledge it wasn't required anywhere.

Maybe we can remove the CUSTOM_SORTING switch and just leave the sec_sort_key
in struct result_note initialized to zero, but it would cost us 4 bytes per
result_note which I wanted to avoid.