pgsql: Repair bug #2839: the various ExecReScan functions need to reset

Log Message:
-----------
Repair bug #2839: the various ExecReScan functions need to reset
ps_TupFromTlist in plan nodes that make use of it.  This was being done
correctly in join nodes and Result nodes but not in any relation-scan nodes.
Bug would lead to bogus results if a set-returning function appeared in the
targetlist of a subquery that could be rescanned after partial execution,
for example a subquery within EXISTS().  Bug has been around forever :-(
... surprising it wasn't reported before.

Tags:
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REL7_4_STABLE

Modified Files:
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    pgsql/src/backend/executor:
        nodeFunctionscan.c (r1.22 -> r1.22.2.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeFunctionscan.c.diff?r1=1.22&r2=1.22.2.1)
        nodeIndexscan.c (r1.84.2.2 -> r1.84.2.3)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeIndexscan.c.diff?r1=1.84.2.2&r2=1.84.2.3)
        nodeResult.c (r1.26 -> r1.26.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeResult.c.diff?r1=1.26&r2=1.26.4.1)
        nodeSeqscan.c (r1.46 -> r1.46.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeSeqscan.c.diff?r1=1.46&r2=1.46.4.1)
        nodeSubqueryscan.c (r1.22 -> r1.22.2.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeSubqueryscan.c.diff?r1=1.22&r2=1.22.2.1)
        nodeTidscan.c (r1.35 -> r1.35.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeTidscan.c.diff?r1=1.35&r2=1.35.2.1)