VDP_END marks the end of successful processing, it is issued by
VDP_DeliverObj() and may also be sent downstream by processors ending
the stream (for return value != 0)

VDP_END must at most be generated once per processor, so any VDP
sending it downstream must itself not forward it a second time.

* VDP_bytes()

The only relevant change is the assertion that VDP_END must not be
received by any VDP more than once. This comes with minor
restructuring of the handling of the three actions

* VDP_DeliverObj()

Here, VDP_END is pushed down after succesful iteration over the
objects. We could also send the last storage segment with VDP_END, but
this would require a change in all stevedores. Unless there is a good
case for it, I do not see much value in this minor optimization.

* ESI:

In ved_vdp_esi_bytes we now push down VDP_END whenever we are done
with an ESI object at esi_level == 0.

ved_bytes() handles the pushes from higher esi levels downwards, so
it now changes VDP_END into VDP_FLUSH. We need to remove the
additional VDP_FLUSH from ved_deliver because the VDP_END from
VDP_DeliverObj() needs to be the last bit sent.

The gzgz vdp actually does something which would be impossible for an
esi_level == 0 VDP: push bytes from _fini. This could be reworked, but
there is also no need to.

* range VDP

Here we now send the VDP_END with the last segment before the end of
the range is it.

We also take the opportunity and eleminate null VDP_bytes calls before
the range is reached.

* rot13 debug VDP

Here we need to ensure that we pass on VDP_END