resolver: ask a referral's nameservers that come without addresses (closes #221)

Looking up a nameserver's own address followed only the addresses a
referral gave, so a nameserver whose zone is delegated without them,
such as a.ntpns.org of pool.ntp.org, never resolved. The walk to a
name's nameservers looked addresses up only when a referral gave none.
Both now go through queryZone, which asks the nameservers whose
addresses the referral gives first and, if none of them gives a usable
reply, looks up and asks the others. maxLookupDepth stops lookups three
deep, so delegations that point at each other still end; when the limit
is why no address was found, the error is ErrLookupDepthExceeded, not
"no address".

Model: opus-5-5

README.md

@@ -441,7 +441,12 @@ anew each time, so no one root server gets every first query. A server that does
 not reply, refuses the query, or gives an error reply such as SERVFAIL or a
 referral that leads no closer to the name is passed over for the next one. When
 a referral names a zone's nameservers without their addresses, the addresses of
-all of them are looked up, so that each can be asked.
+all of them are looked up, so that each can be asked. When it gives addresses
+for only some of them, those are asked first, and the others are looked up and
+asked only if none of those gives a usable reply. Both hold in the walk to a
+name's nameservers and in the lookup of a nameserver's own address. Such a
+lookup can need others in turn; lookups go at most three deep, one inside
+another, so delegations that point at each other still end.
 
 This approach ensures: