Fix mergejoin cost estimation so that we consider the statistical ranges of

the two join variables at both ends: not only trailing rows that need not be
scanned because there cannot be a match on the other side, but initial rows
that will be scanned without possibly having a match.  This allows a more
realistic estimate of startup cost to be made, per recent pgsql-performance
discussion.  In passing, fix a couple of bugs that had crept into
mergejoinscansel: it was not quite up to speed for the task of estimating
descending-order scans, which is a new requirement in 8.3.