Fix up LIMIT/OFFSET planning so that we cope with non-constant LIMIT

or OFFSET clauses by using estimate_expression_value().  The main advantage
of this is that if the expression is a Param and we have a value for the
Param, we'll use that value rather than defaulting.  Also, fix some
thinkos in the logic for combining LIMIT/OFFSET with an externally
supplied tuple fraction (this covers cases like EXISTS(...LIMIT...)).
And make sure the results of all this are shown by EXPLAIN.  Per a
gripe from Merlin Moncure.