Fix code that checks to see if an index can be considered to match the query's

Fix code that checks to see if an index can be considered to match the query's
requested sort order.  It was assuming that build_index_pathkeys always
generates a pathkey per index column, which was not true if implied equality
deduction had determined that two index columns were effectively equated to
each other.  Simplest fix seems to be to install an option that causes
build_index_pathkeys to support this behavior as well as the original one.
Per report from Brian Hirt.