6th rebase of NTP mode6 support (f-NTP-mode6)

[sjvudp]

As my previous pull request(s) all seem to have stalled, I decided to rebase on commit f085c93f and start another (this!) pull request. Features are implementing NTP mode6 packet decoding. Due to the long time since starting the branch (and due to the fact that this is my first github pull contribution) the branch may be a bit more complex than necessary. If you want, I can provide one big commit instead. I don't know whether I'll have time until next year to do major work on this, but the NTP "test suite" should be condensed to fewer, more essential packets. The packets used now are more or less "ad hoc" captures to have "something" (which is "better than nothing" TM).