vrrp: add auth_hmac receive-only mode

A node reloaded into permissive signs toward peers that cannot
accept the trailer yet, so a slow rollout risked dual active.
receive-only verifies without signing, which makes the receive-only,
permissive, enforce migration hitless. Suggested by Aditya Dogra.

Signed-off-by: Alexandre Cassen <acassen@gmail.com>