pgsql: Refactor SPI_cursor_open/SPI_cursor_open_with_args so that the

Log Message:
-----------
Refactor SPI_cursor_open/SPI_cursor_open_with_args so that the latter sets
the PARAM_FLAG_CONST flag on the parameters that are passed into the portal,
while the former's behavior is unchanged.  This should only affect the case
where the portal is executing an EXPLAIN; it will cause the generated plan to
look more like what would be generated if the portal were actually executing
the command being explained.  Per gripe from Pavel.

Modified Files:
--------------
    pgsql/src/backend/executor:
        spi.c (r1.195 -> r1.196)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c?r1=1.195&r2=1.196)