pgsql: Use a hopefully-more-reliable method of detecting default

Log Message:
-----------
Use a hopefully-more-reliable method of detecting default selectivity
estimates when combining the estimates for a range query.  As pointed out
by Miquel van Smoorenburg, the existing check for an impossible combined
result would quite possibly fail to detect one default and one non-default
input.  It seems better to use the default range query estimate in such
cases.  To do so, add a check for an estimate of exactly DEFAULT_INEQ_SEL.
This is a bit ugly because it introduces additional coupling between
clauselist_selectivity and scalarltsel/scalargtsel, but it's not like
there wasn't plenty already...

Modified Files:
--------------
    pgsql/src/backend/optimizer/path:
        clausesel.c (r1.70 -> r1.71)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/clausesel.c.diff?r1=1.70&r2=1.71)
    pgsql/src/backend/utils/adt:
        selfuncs.c (r1.166 -> r1.167)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.166&r2=1.167)
    pgsql/src/include/utils:
        selfuncs.h (r1.19 -> r1.20)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/selfuncs.h.diff?r1=1.19&r2=1.20)