Unsupported effective_io_concurrency platforms

If a platform doesn't have posix_fadvise(), we don't allow
effective_io_concurrency to be set to anything but zero:
test=> set  effective_io_concurrency = 1;ERROR:  1 is outside the valid range for parameter "effective_io_concurrency"
(0.. 0)test=> set  effective_io_concurrency = 0;SET
 

Do we want to give a more informative error message, like "not supported
on this platform?"

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
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