Use Secure Neighbor Discovery to protect against attacks

To protect against ARP poisoning and other attacks against NDP functions, Secure Neighbor Discovery (SEND) Protocol should be deployed where preventing access to the broadcast segment might not be possible. SEND uses RSA key pairs to produce Cryptographically Generated Addresses (CGA), as defined in RFC 3972. This ensures that the claimed soruce of an NDP message is the owner of the claimed address.

From RFC 4861 Section 11.1. Threat Analysis:
```
This section discusses the main threats associated with Neighbor
Discovery. A more detailed analysis can be found in [PSREQ]. The
main vulnerabilities of the protocol fall under three categories:

- Denial-of-Service (DoS) attacks.
- Address spoofing attacks.
- Router spoofing attacks.

[...]

The protocol contains no mechanism to determine which neighbors are
authorized to send a particular type of message (e.g., Router
Advertisements); any neighbor, presumably even in the presence of
authentication, can send Router Advertisement messages thereby being
able to cause denial of service. Furthermore, any neighbor can send
proxy Neighbor Advertisements as well as unsolicited Neighbor
Advertisements as a potential denial-of-service attack.
```

This issue homes the discussion of whether or not we need this feature.