we take the chance of the breaking change in
681c1199 to also add a VRT_CTX
argument to vmod_priv_fini_f. This allows a vmod_priv fini function,
for example, access to the request or simpler and better logging
(because the task's vsl buffer can be used where otherwise only
unbuffered vsl was possible).

    Implementation:

The meat of this commit really only is the change to the
vmod_priv_fini_f typedef and VRT_priv_fini().

All other changes are either to bundled vmods or the straight forward
infrastructure to make available a ctx to VRT_priv_fini(), which, for
client and backend context, is called via VCL_TaskLeave().

Consequently, it made sense to also change the signature
Req_Rollback() and Bereq_Rollback().