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.
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.

Chibin/gpdb,arcivanov/postgres-xl,yuanzhao/gpdb,yazun/postgres-xl,jmcatamney/gpdb,adam8157/gpdb,Chibin/gpdb,greenplum-db/gpdb,snaga/postgres-xl,edespino/gpdb,pavanvd/postgres-xl,arcivanov/postgres-xl,jmcatamney/gpdb,ashwinstar/gpdb,adam8157/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,postmind-net/postgres-xl,lisakowen/gpdb,yuanzhao/gpdb,postmind-net/postgres-xl,yuanzhao/gpdb,oberstet/postgres-xl,Postgres-XL/Postgres-XL,yuanzhao/gpdb,ovr/postgres-xl,ashwinstar/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,Postgres-XL/Postgres-XL,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,Chibin/gpdb,oberstet/postgres-xl,ashwinstar/gpdb,yazun/postgres-xl,Postgres-XL/Postgres-XL,greenplum-db/gpdb,Chibin/gpdb,50wu/gpdb,xinzweb/gpdb,tpostgres-projects/tPostgres,50wu/gpdb,zeroae/postgres-xl,tpostgres-projects/tPostgres,adam8157/gpdb,greenplum-db/gpdb,arcivanov/postgres-xl,edespino/gpdb,yazun/postgres-xl,tpostgres-projects/tPostgres,techdragon/Postgres-XL,edespino/gpdb,arcivanov/postgres-xl,ovr/postgres-xl,ovr/postgres-xl,ashwinstar/gpdb,edespino/gpdb,ovr/postgres-xl,jmcatamney/gpdb,kmjungersen/PostgresXL,lisakowen/gpdb,oberstet/postgres-xl,arcivanov/postgres-xl,adam8157/gpdb,yuanzhao/gpdb,zeroae/postgres-xl,snaga/postgres-xl,jmcatamney/gpdb,ashwinstar/gpdb,tpostgres-projects/tPostgres,techdragon/Postgres-XL,techdragon/Postgres-XL,yazun/postgres-xl,jmcatamney/gpdb,lisakowen/gpdb,jmcatamney/gpdb,arcivanov/postgres-xl,yuanzhao/gpdb,adam8157/gpdb,postmind-net/postgres-xl,lisakowen/gpdb,edespino/gpdb,oberstet/postgres-xl,Chibin/gpdb,ashwinstar/gpdb,ovr/postgres-xl,ashwinstar/gpdb,tpostgres-projects/tPostgres,Chibin/gpdb,postmind-net/postgres-xl,adam8157/gpdb,Postgres-XL/Postgres-XL,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,kmjungersen/PostgresXL,lisakowen/gpdb,Postgres-XL/Postgres-XL,Chibin/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,pavanvd/postgres-xl,edespino/gpdb,edespino/gpdb,xinzweb/gpdb,kmjungersen/PostgresXL,oberstet/postgres-xl,adam8157/gpdb,50wu/gpdb,50wu/gpdb,snaga/postgres-xl,pavanvd/postgres-xl,greenplum-db/gpdb,techdragon/Postgres-XL,Chibin/gpdb,zeroae/postgres-xl,yuanzhao/gpdb,techdragon/Postgres-XL,postmind-net/postgres-xl,kmjungersen/PostgresXL,edespino/gpdb,edespino/gpdb,adam8157/gpdb,kmjungersen/PostgresXL,50wu/gpdb,zeroae/postgres-xl,jmcatamney/gpdb,lisakowen/gpdb,snaga/postgres-xl,Chibin/gpdb,snaga/postgres-xl,yazun/postgres-xl,lisakowen/gpdb,zeroae/postgres-xl,jmcatamney/gpdb,pavanvd/postgres-xl,yuanzhao/gpdb,edespino/gpdb,xinzweb/gpdb,Chibin/gpdb,pavanvd/postgres-xl,yuanzhao/gpdb