Check transition of circuit purpose from INTRO->GENERAL if nodes are constrained

Check transition of circuit purpose from INTRO->GENERAL if nodes are constrained

This looked at first like another fun way around our node selection
logic: if we had introduction circuits, and we wound up building too
many, we would turn extras into general-purpose circuits.  But when we
did so, we wouldn't necessarily check whether the general-purpose
circuits conformed to our node constraints.  For example, the last
node could totally be in ExcludedExitNodes and we wouldn't have cared...

...except that the circuit should already be internal, so it won't get user
streams attached to it, so the transition should generally be allowed.
Add an assert to make sure we're right about this, and have it not
check whether ExitNodes is set, since that's irrelevant to internal
circuits.