Various cleanups: threads, STA_NANO

[Hal Murray]

> I think we covered this in https://gitlab.com/NTPsec/ntpsec/issues/421

I think that's a different issue.  OpenBSD doesn't support that API at all.  
The STA_NANO case I'm interested in is several ifdef-s in ntp_loopcontrol  No 
big deal, just some clutter we might be able to remove.

The question is: Do any of the systems we want to support have an old version 
of ntp_adjtime, old enough that it doesn't know about STA_NANO.  If so, I'd 
like to collect a list.


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