Pithy patch for more detailed error reporting when effective_io_concurrency is set to nonzero on platforms lacking posix_fadvise()

Per Tom's suggestion on bug #15396, here's a patch to have platforms such as OSX give a more descriptive message when rejecting a nonzero value for effective_io_concurrency.

I had to adjust the GUC's wiring in the #ifndef case so that check_effective_io_concurrency() would be called when a nonzero value is supplied instead of just short-circuiting in parse_and_validate_value() when outside of [conf->min, conf->max].

James