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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REL8_0_STABLE

Modified Files:
--------------
    pgsql/src/backend/executor:
        nodeFunctionscan.c (r1.29 -> r1.29.4.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeFunctionscan.c.diff?r1=1.29&r2=1.29.4.1)
        nodeIndexscan.c (r1.99.4.1 -> r1.99.4.2)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeIndexscan.c.diff?r1=1.99.4.1&r2=1.99.4.2)
        nodeResult.c (r1.29 -> r1.29.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeResult.c.diff?r1=1.29&r2=1.29.4.1)
        nodeSeqscan.c (r1.51 -> r1.51.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeSeqscan.c.diff?r1=1.51&r2=1.51.4.1)
        nodeSubqueryscan.c (r1.25 -> r1.25.4.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeSubqueryscan.c.diff?r1=1.25&r2=1.25.4.1)
        nodeTidscan.c (r1.41 -> r1.41.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeTidscan.c.diff?r1=1.41&r2=1.41.4.1)