pgsql: Fix oversight in planning for multiple indexscans driven by

Log Message:
-----------
Fix oversight in planning for multiple indexscans driven by
ScalarArrayOpExpr index quals: we were estimating the right total
number of rows returned, but treating the index-access part of the
cost as if a single scan were fetching that many consecutive index
tuples.  Actually we should treat it as a multiple indexscan, and
if there are enough of 'em the Mackert-Lohman discount should kick in.

Modified Files:
--------------
    pgsql/src/backend/optimizer/path:
        costsize.c (r1.159 -> r1.160)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/costsize.c.diff?r1=1.159&r2=1.160)
    pgsql/src/backend/utils/adt:
        selfuncs.c (r1.207 -> r1.208)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.207&r2=1.208)
    pgsql/src/include/utils:
        selfuncs.h (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/selfuncs.h.diff?r1=1.33&r2=1.34)