Blender Git Loki

Two constraints related fixes
* Bugfix #8599d:
When using the ChildOf constraint on bones without parents, with cyclic-offset in action the effect of the cyclic-offset was applied twice. Added a bone option to make the effect of cyclic-offset not be applied to rootbones which have the option 'enabled'.

In the UI, this is presented in the opposite way. It is represented by the 'Offs' button beside the parent-bone selector in the Armature Bones panel when in EditMode.

* Head/Tail setting and Constraint Channels:
Added back in checks to see if the head/tail IPO-curves for Constraint Channels can get applied for the target constraint. I had removed these when applying the patch, but I'd overlooked the fact that users could add an IPO-curve for this from the IPO-editor, potentially mucking up the behaviour of some constraints.