vchoi_fork

[svn-r7875] Purpose:
    Omnibus floating-point bug fix changes

Description:
    There are a number of problems in the floating-point conversion code that
were exposed by Ray's recent int<->float checkin:
    - The 'my_isnan' code in test/dtypes.c was broken and would always return
        true.  The meant that the actual values in the float<->float conversion
        tests were _never_ checked, hiding the other bugs included in this
        checkin.
    - A recent change I made to the type conversion code used "FLT_MIN" instead
        of "-FLT_MAX" for the most negative 'float' value for the double->float
        conversion, which meant that any the negative number that was converted
        from a double to a float would have been mapped to zero, essentially.
    - A change that Robb appeared to have made ~2.5 years ago to the "generic"
        float->float conversion routine appears to be incorrect and I've backed
        it out.
    - Floating-point conversions on SGI's which converted denormalized values
        would be mapped to zero instead of being propertly preserved in the new
        type.  This was addressed by an SGI-specific system call to prevent the
        behavior.

Solution:
    Described above, generally.

Platforms tested:
    FreeBSD 4.9 (sleipnir)
    h5committest

Misc. update:
    release_docs/RELEASE update forthcoming...