DNS cleanup

[Eric S. Raymond]

Hal Murray <hmurray at megapathdsl.net>:
> Do we want a configure option to build/run without DNS lookups?  Even if we 
> don't, we should probably structure the code to support that.

Agreed with the second.  But we've got enough obscure seldom-used
options that I'm strongly against adding another exposed one without
demonstrated need.

> My plan is to delete ntp_intres, ntp_worker, and work_thread,
> then add a new module to fill in the gaps.

I like that plan.

> Eric:
>   close_all_except is in libntp/ntp_worker.c
>   where should it live?  Or can we get rid of it?
> It's only called from ntpdmain.

Move it to ntpdmain.c for now, then.  It's possible we can get rid of it, but
I'd rather not have that change be entangled with your refactor.

Be aware that at some point I would still be interested in replacing
our internal async-lookup code with calls to c_ares or equivalent, on
the general principle that it's better when specialty code unrelated
to our business logic is somebody else's maintainance problem.

However, *don't let that stop you*.  Your change might make alter the
tradeoffs, making plugging in c_ares unnecessry.  If not, I have
no doubt your change will make it easier.
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