pgsql: Fix a couple of places in execMain that erroneously assumed that

Log Message:
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Fix a couple of places in execMain that erroneously assumed that SELECT FOR
UPDATE/SHARE couldn't occur as a subquery in a query with a non-SELECT
top-level operation.  Symptoms included outright failure (as in report from
Mark Mielke) and silently neglecting to take the requested row locks.

Back-patch to 8.3, because the visible failure in the INSERT ... SELECT case
is a regression from 8.2.  I'm a bit hesitant to back-patch further given the
lack of field complaints.

Tags:
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REL8_3_STABLE

Modified Files:
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    pgsql/src/backend/executor:
        execMain.c (r1.303 -> r1.303.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execMain.c?r1=1.303&r2=1.303.2.1)