* doublest.c: Improve comments a bit.

* doublest.c: Improve comments a bit.
(floatformat_from_length): New function.
(NAN): Define to 0.0 if not already defined.
(extract_floating): Rewrite to use floatformat_from_length.  Warn
instead of error if LEN doesn't match a known floating-point type,
and return NaN (or 0.0 if NaN isn't available) in that case.
(store_floating): Likewise, but zero out the target byte-stream if
LEN doesn't match a known floating-point type.
(extract_typed_floating): Reformat a bit.
(store_typed_floating): Reformat a bit.  Add comment about zeroing
out padding in the target buffer.
* doublest.h (extract_floating, store_floating): Fix comment about
deprecation of these functions.  Add parameter names to prototypes.