MFC r324119:

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.