This is the refactoring part of D8805 (should be no functional changes).
- exposes pose-related part of former 'get_constraints()' from
interface_templates.c to new ED_object_pose_constraint_list
- rename ED_object_constraint_list_from_context -->
ED_object_constraint_active_list
Also clarify comments on both of these.
ref T80464
ref https://developer.blender.org/D8805
9dcae4eb17