check_nonce

Check nonce validity in a request and response. Return value reflects result:

1: nonces present and equal.
2: nonces both absent.
3: nonce present in response only.
0: nonces both present and not equal.

-1: nonce in request only.

For most responders clients can check return > 0.
If responder doesn't handle nonces return != 0 may be
necessary. return == 0 is always an error.