Drop PRIV_SESS, as agreed on VDD14Q4.
Make the VRTPRIV_* interface slightly more generic to let us use it without needing a req/sess pointer.
Reintroduce PRIV_REQ as PRIV_TASK. PRIV_TASK is available (as separate priv pointers) both in the client side and the backend side VCL functions.
Merge branch 'vmodpriv'