Change the definition of the `PCHAN_HAS_CONST` pose channel flag. It used
to mean "has a constraint that is not IK or Spline IK", and now it just
means "has a constraint".
This has no direct effect yet, as the flag is only used in drawing code
when there is no (spline) IK constraint. However, I feel that the flag
name should match its expected behaviour, and IMO, in this case, making
that behaviour simpler is better than documenting more.
05b725c744