Cleanup: use BKE_fcurve_ prefix for keyframing bezier functions

- BKE_bezt_subdivide_handles -> BKE_fcurve_bezt_subdivide_handles
- binarysearch_bezt_index -> BKE_fcurve_bezt_binarysearch_index

These functions are specific to F-Curves and don't make sense for other
uses of BezTriple (curve-object data for e.g.)

Also:

- Move detailed doxygen comment above code, following code-style.
- Mark bezt_add_to_cfra_elem unused.

source/blender/python/intern/bpy_rna_anim.c

@@ -498,7 +498,7 @@ PyObject *pyrna_struct_keyframe_delete(BPy_StructRNA *self, PyObject *args, PyOb
         int i;
 
         /* try to find index of beztriple to get rid of */
-        i = binarysearch_bezt_index(fcu->bezt, cfra, fcu->totvert, &found);
+        i = BKE_fcurve_bezt_binarysearch_index(fcu->bezt, cfra, fcu->totvert, &found);
         if (found) {
           /* delete the key at the index (will sanity check + do recalc afterwards) */
           delete_fcurve_key(fcu, i, 1);