Interface iteration

[Hal Murray]

While looking for old mail tangled with #690, I stumbled into this, from Eric. 
 Did we ever do anything with this?  This seems like a wonderful opportunity.

The catch may be that ntpd needs to know which interface a packet arrived on, 
actually the destination IP address.

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You missed some interesting discussion on IRC last night (for after
release).  While working on the CVEs Daniel has spotted that all the
iteration-over-interfaces stuff (which is 75% of ntp_io.c and almost
all of what's left of the ISC library) is unnecessary if we trust
IPv4/IPv6 wildcard addresses to work (which are POSIX).

This is kind of a big deal not just because of the LOC reduction but
because it would let us sever a major and several minor platform
dependencies - not just getifaddrs but the routing-sockets stuff as
well.


-- 
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