pgsql: Prevent ExecInsert() and ExecUpdate() from scribbling on the

Log Message:
-----------
Prevent ExecInsert() and ExecUpdate() from scribbling on the result tuple
slot of the topmost plan node when a trigger returns a modified tuple.
These appear to be the only places where a plan node's caller did not
treat the result slot as read-only, which is an assumption that nodeUnique
makes as of 8.1.  Fixes trigger-vs-DISTINCT bug reported by Frank van Vugt.

Modified Files:
--------------
    pgsql/src/backend/executor:
        execMain.c (r1.256 -> r1.257)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execMain.c.diff?r1=1.256&r2=1.257)
        execUtils.c (r1.126 -> r1.127)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execUtils.c.diff?r1=1.126&r2=1.127)
    pgsql/src/include/nodes:
        execnodes.h (r1.139 -> r1.140)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/execnodes.h.diff?r1=1.139&r2=1.140)