[clean up the variety of frexp(), ldexp() and modf()]

rpaulo@fnop.net said:
> Is this something that we should add to htdocs/developers/features ? 

I don't thing so. It is just a minor cleanup, and there are more
cases where we have some duplication between libc and libm
(eg fabs and (is)nanf). We can't add a column to the feature table
for all of these.

best regards
Matthias