force fcurve AUTO handlers to use 1/3 of the distance for smoothness
see: http://www.blender.org/forum/viewtopic.php?p=65508&sid=02e88fe6e3a43338ec8bc354991bc402

Note: this is also used by the curve code. The magic number 2.5614f
comes from review 290361776e5858b3903a83c0cddf722b8340e699