pgsql: Fix ExecMakeTableFunctionResult to verify that all rows returned

Log Message:
-----------
Fix ExecMakeTableFunctionResult to verify that all rows returned by a SRF
returning "record" actually do have the same rowtype.  This is needed because
the parser can't realistically enforce that they will all have the same typmod,
as seen in a recent example from David Wheeler.

Back-patch to 8.0, which is as far back as we have the notion of RECORD
subtypes being distinguished by typmod.  Wheeler's example depends on
8.4-and-up features, but I suspect there may be ways to provoke similar
failures before 8.4.

Modified Files:
--------------
    pgsql/src/backend/executor:
        execQual.c (r1.264 -> r1.265)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execQual.c?r1=1.264&r2=1.265)