We use shard_param_task_{r,l} on (struct sharddir *) and (struct
vmod_directors_shard_param *), so the id parameter is (const void *).
Yet we still want to make sure that we always retrieve the intended
PRIV_TASK, and we can by simply asserting that the vcl_name attribute
matches.

Note that a simple pointer comparison is sufficient here because,
unltimately, the PRIV_TASK vcl_name is always that of a vmod object,
assigned by the constructor.

This brings back the "who" parameter from
9ea2f29f, but used differently.