MFC r324119:

• Update function definitions.
• Ensure that the datalen always describes the length after the IPv6 header consistently, not matter which protocol us used for probes..
• Document that the default length is 20, not 12.
• Don't send inormation in probe packets which is not needed or even checked when the responses are processed.
• Address CID 978587.

This is mainly a cleanup preparing the addition of SCTP and TCP
as possible probe packet protocols.