Use guc.c's parse_int() instead of pg_atoi() to parse fillfactor in

Use guc.c's parse_int() instead of pg_atoi() to parse fillfactor in
default_reloptions().  The previous coding was really a bug because pg_atoi()
will always throw elog on bad input data, whereas default_reloptions is not
supposed to complain about bad input unless its validate parameter is true.
Right now you could only expose the problem by hand-modifying
pg_class.reloptions into an invalid state, so it doesn't seem worth
back-patching; but we should get it right in HEAD because there might be other
situations in future.  Noted while studying GIN fast-update patch.