[Tinymux] Adding Penn's fmod function

[Suzanne Baylor]

I'm copying some code over from Penn to TinyMUX 2.3.5.24 which uses the 
fmod function from Penn which apparantly isn't supported in TinyMUX. Is 
there an easy way to add this in or code around it? My last math class 
was 20 years ago so I've no idea how to fake one up myself :) I tried 
just pasting the hc into functions.cpp but it complains a ton on compile 
about missing identifiers, etc (not surprising.)

---
fmod(<number>,<divisor>)

  Identical to mod() but may take floating point arguments. The return
  value is the (possibly floating point) smallest positive remainder
  left after subtracting the largest number of multiples of <divisor>
  from <number>.

  Example:
  > think fmod(6.1,2.5)
  1.1

(above copyright Javelin and crew)