Try to be more consistent about accepting denormalized float8 numbers.

Try to be more consistent about accepting denormalized float8 numbers.

On some platforms, strtod() reports ERANGE for a denormalized value (ie,
one that can be represented as distinct from zero, but is too small to have
full precision).  On others, it doesn't.  It seems better to try to accept
these values consistently, so add a test to see if the result value
indicates a true out-of-range condition.  This should be okay per Single
Unix Spec.  On machines where the underlying math isn't IEEE standard, the
behavior for such small numbers may not be very consistent, but then it
wouldn't be anyway.

Marti Raudsepp, after a proposal by Jeroen Vermeulen