Handle graceful shudown

[christophefontaine]

Description

When we quit grout (with ctrl+c), the other network elements do not know that.
In order to be kind with our neighbor routers, we need a way to send packets when we quit.

A generic event could be added, and triggered when we quit grout.
+ EVENT_GRACEFUL_SHUTDOWN = EVENT_TYPE(GR_INFRA_MODULE, 0xf000),
And we'd have to wait for the registered callbacks to be triggered before quitting.

Context and Use Cases

[6.2.5](https://www.rfc-editor.org/rfc/rfc4861#section-6.2.5).  Ceasing To Be an Advertising Interface

      - shutting down the system.
   In such cases, the router SHOULD transmit one or more (but not more
   than MAX_FINAL_RTR_ADVERTISEMENTS) final multicast Router
   Advertisements on the interface with a Router Lifetime field of zero.

While we don't break any RFC (SHOULD, not MUST), we should comply with it :)