Minor code improvements to create_foreignscan_plan/ExecInitForeignScan

To save cycles, I modified create_foreignscan_plan so that it detects
whether any system columns are requested if scanning a base relation.
Also, I revised other code there a little bit.

For ExecInitForeignScan, I simplified the code there to determine the
scan tuple type, whith seems to me complex beyound necessity.  Maybe
that might be nitpicking, though.

Best regards,
Etsuro Fujita