Have descend return the radix level we stopped at
Have descend return the radix level we stopped at

Rather awkward API, but it's enough to implement skipping of empty
nodes.

bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6