This is a different fix for the issue from D2088, preserving backwards compatibility
for IK stretching. The main problem with this patch is that this new behavior has
been there for a year, so it may break rigs created since then which rely on the new
IK stretch behavior.

Test file for various cases:
https://developer.blender.org/diffusion/BL/browse/trunk/lib/tests/animation/IK.blend