VDI_Resolve -> VRT, default behaviour for no healthy() cb

[Nils Goroll]

I'm picking up two loose threads from #varnish-hacking:

VDI_Resolve
-----------

I'm working on a director which needs to resolve another (layered) director
argument, so I'd prefer to have basically VDI_Resolve in VRT. Proposed signature:

	VCL_BACKEND VRT_Resolve(VRT_CTX, VCL_BACKEND)

the VCL_BACKEND argument would be used instead of bo->director_req.
Also I'd want to move the logging calls to the caller.

avoid director healthy() callback
---------------------------------

To avoid code duplication, I think core code should basically call

	VRT_Healthy(VRT_Resolve(ctx, backend));

if there is no healthy callback. IMHO this would make much more sense than just
asserting a director without a healthy callback to be healthy always.

Thx, Nils