Transports should be free to keep a reference on the object to be
delivered until after their transport function returns, but
HSH_Cancel() in cnt_transmit() prevented the object from being of any
use for the case that it is final (pass/hfm/hfp).

We solve this by moving the HSH_Cancel() close to VDP_Close, which
also makes sense from the perspective of the VDP design: Until the VDP
close, filters could still reference object data.

HSH_Cancel() needs the objcore, which could be reachable also via
vdc->req. But that member is unset in VDP_DeliverObj(), presumably to
make it clear that VDP .bytes callbacks should not access request
data. Thus, we pass it as a new argument to VDP_Close() as well as any
boc being held by the caller.

The objcore can also be NULL for the case where a transport generates
the body without holding an objcore at all.