Comment tweak.
NVgf/NVff agree with -Duselongdouble.
Make sprintf %c and chr() on inf/nan return the U+FFFD.

%c was made to produce "Inf"/"NaN" earlier, but let's
keep with the Unicode way, and make chr() agree with %c.
pack c/C on inf/nan.

Made them return the 0xFF byte (and warn).  Not necessarily the best
choice, but there's not that much room in just 256 bytes for all of
the inf/-inf/nan.  This same choice will need to be made with wider
integer packs.