to avoid using stale pointers after a rollback.

Before this change, we would call VRT_priv_* only once per subroutine,
which can be *) a nice performance optimization, but leaves us with stale
pointers after a rollback.

Rather than adding complications for the rollback case just to keep the
option of the "per subroutine pointer cache", just retrieve a fresh
priv pointer every time.

The other use of the per subroutine initialization was error handling,
which needs additional code outside the function arguments, simply
because a return statement is not possible within function arguments. We
removed the requirement for error handling in the previous commit by
making sure that VRT_priv_{task,top} always return a valid pointer.

Fixes https://github.com/varnish/varnish-modules/issues/222

Alternative implementation to #4060

*) It is not an optimization in all cases, for example the priv pointers
   were intialized unconditionally, even if code using them was not
   reached - but then again, this is something C compilers might
   optimize...