index tuple (logical position within A LEVEL). bti_oid & bti_dummy
taken off from BTItemData.
2. Fix for multi-column indices (nbtsearch.c):
   _bt_binsrch() - for searches on internal pages having keysize <
	number of attrs we point at the last item < the scankey, not at the
	first item = the scankey;
   _bt_moveright() - if keysize < number of attrs we compare scankey with
	_last_ item on current page to decide should we move right or
	not.