[Cfp-interest] exceptions and flags

[David Hough CFP]

One point to be remembered in considering reference implementations based
on flags is that you can't detect underflow exceptions just by checking
the underflow flag.    Even in 754-1985 there's this little asymmetry and
while there are reasons for it, "easy to remember" is not one of
them.

So detecting exact underflows in all cases is a bit complicated.    Each
such result has to be detected by checking for subnormal results somehow.
It's a lot easier to enable the underflow trap.