This matches the behavior of other applications and improves UX,
especially for new users.
The former behaviour was an especially annoying idiosyncracy for
Blender. Maya in particular makes heavy use of the Parent
constraint in its workflow, and sets the inverse automatically.
This commit will improve the workflow for Maya artists who are
transitioning to Blender.
I don't know if my approach here is "correct". Some constraints
are initialized with correct settings already, such as the
Stretch-To constraint (original length) or the Limit Location
constraint (distance). I didn't check how those constraints
get or set this information... instead, I added a few lines to
constraint_add_exec that sets the CHILDOF_SET_INVERSE flag
if the new constraint is a Child-Of. This seems safe, since
the user is probably adding the constraint in pose-mode
already. However, I don't know if this is the correct approach.