Change the planner to allow indexscan qualification clauses to use

Change the planner to allow indexscan qualification clauses to use
nonconsecutive columns of a multicolumn index, as per discussion around
mid-May (pghackers thread "Best way to scan on-disk bitmaps").  This
turns out to require only minimal changes in btree, and so far as I can
see none at all in GiST.  btcostestimate did need some work, but its
original assumption that index selectivity == heap selectivity was
quite bogus even before this.