Only one successful exit from grok_infnan().
Comment why hexfp is set here, not earlier.
Simplify the hexfp space computation.

And prepare for the double double case being properly implemented.
Post-VAX VMS has fp_classify().
*fp*class* comment tweaks.
Avoid useless warning, remove debug code.
Add more infnan tests.
printf inf/nan should be inf/nan.

Before: printf %[ducp] for Inf/NaN produced quite surprising results:
1, 0, -1, 18446744073709551615, 9223372036854775808, -9223372036854775807,
bogus Unicode code points, random heap addresses in hex.
Detect false infinities.