Replace a proliferation of buggy MD implementations of modf() with a

Replace a proliferation of buggy MD implementations of modf() with a
working MI one.  The MI one only needs to be overridden on machines
with non-IEEE754 arithmetic.  (The last supported one was the VAX.)
It can also be overridden if someone comes up with a faster one that
actually passes the regression tests -- but this is harder than it sounds.