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.

Modified Files:
--------------
pgsql/src/backend/executor:
nodeBitmapHeapscan.c (r1.14 -> r1.15)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeBitmapHeapscan.c.diff?r1=1.14&r2=1.15)
nodeFunctionscan.c (r1.40 -> r1.41)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeFunctionscan.c.diff?r1=1.40&r2=1.41)
nodeIndexscan.c (r1.118 -> r1.119)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeIndexscan.c.diff?r1=1.118&r2=1.119)
nodeResult.c (r1.34 -> r1.35)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeResult.c.diff?r1=1.34&r2=1.35)
nodeSeqscan.c (r1.61 -> r1.62)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeSeqscan.c.diff?r1=1.61&r2=1.62)
nodeSubqueryscan.c (r1.32 -> r1.33)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeSubqueryscan.c.diff?r1=1.32&r2=1.33)
nodeTidscan.c (r1.51 -> r1.52)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeTidscan.c.diff?r1=1.51&r2=1.52)
nodeValuesscan.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeValuesscan.c.diff?r1=1.3&r2=1.4)