VCC backend functions should need busyobject argument

[Nils Goroll]

Just a quick note on something I noticed working with Dag on vmod_saintmode in
the context of other directors (I am dropping it knowing that this is not
particularly precise just to not loose it):

The director VMOD backend functions can return their own object, in which case
the resolve function will choose the "real" backend when an actual backend
request is about to be issued. Alternatively, they can resolve (choose a
backend) immediately and return a reference to a "real" backend object (or the
next level down).

For the latter way, there is no struct busyobj passed down, and we probably need
to change that - otherwise stacking directors with vmod_saintmode won't work.

Nils