What packet modes do we support?

[Hal Murray]

> The page that covers differences from Classic - docs/ntpsec.txt.  It's under
> Security. 

Thanks.

* peer mode has been removed.  The keyword peer in ntp.conf is now
  just an alias for keyword server.

* Broadcast- and multicast client modes, which are impossible to
  secure, have been removed. Broadcast (but not multicast) service can
  still be enabled, though this is a deprecated and unsupported mode
  of operation and may be entirely removed in a future release.

---------

I think that means that ephemeral mode is no longer used.  (It used to be 
tangled up with using the pool as well as peer and broadcast client.)  That 
will allow a layer of cleanups throughout the documentation - mostly 
deletions.

That greatly simplifies the receive packet processing.  The MATCH_ASSOC macro 
and its table in the top of ntp_peer can be dropped.

We only expect 3 modes of packets:
  Mode 6 gets diverted off to separate logic very early.
  Mode 3 is requests.  We answer them.
  Mode 4 is responses to our requests.

Do we want to process mode 1?  (Other end is trying to peer with us.)
I think it works to answer them right now, without setting up a peer block to 
save state.

-- 
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