pgsql: Fix cost_mergejoin's failure to adjust for rescanning of

Log Message:
-----------
Fix cost_mergejoin's failure to adjust for rescanning of non-unique merge join
keys when considering a semi or anti join. This requires estimating the
selectivity of the merge qual as though it were a regular inner join condition.
To allow caching both that and the real outer-join-aware selectivity, split
RestrictInfo.this_selec into two fields.

This fixes one of the problems reported by Kevin Grittner.

Modified Files:
--------------
pgsql/src/backend/nodes:
copyfuncs.c (r1.422 -> r1.423)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c?r1=1.422&r2=1.423)
outfuncs.c (r1.351 -> r1.352)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/outfuncs.c?r1=1.351&r2=1.352)
pgsql/src/backend/optimizer/path:
clausesel.c (r1.96 -> r1.97)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/clausesel.c?r1=1.96&r2=1.97)
costsize.c (r1.203 -> r1.204)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/costsize.c?r1=1.203&r2=1.204)
equivclass.c (r1.16 -> r1.17)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/equivclass.c?r1=1.16&r2=1.17)
orindxpath.c (r1.86 -> r1.87)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/orindxpath.c?r1=1.86&r2=1.87)
pgsql/src/backend/optimizer/prep:
prepunion.c (r1.164 -> r1.165)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepunion.c?r1=1.164&r2=1.165)
pgsql/src/backend/optimizer/util:
restrictinfo.c (r1.56 -> r1.57)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/restrictinfo.c?r1=1.56&r2=1.57)
pgsql/src/include/nodes:
relation.h (r1.167 -> r1.168)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/relation.h?r1=1.167&r2=1.168)