resolver: store a name's CNAME once per nameserver (closes #220)

For a name with a CNAME, a nameserver answers a query of any type with
that CNAME, and the records of every answer were added, so the CNAME
was stored once for each of the eight record types asked for.
collectAnswerRecords now adds each value once per record type.

A state file saved before this holds the repeated values. Load keeps
each record value once, so the first check after upgrading sees no
record change and notifies nothing for them.

Model: opus-5-5

internal/resolver/iterative_test.go

@@ -238,6 +238,38 @@ func TestExtractRecordValue_LetterCase(t *testing.T) {
 	}
 }
 
+// TestCollectAnswerRecords_CNAMEOnce collects the answers a nameserver
+// gives for a name with a CNAME, one for each record type a check asks
+// for. Each answer holds the CNAME, which must be stored once.
+func TestCollectAnswerRecords_CNAMEOnce(t *testing.T) {
+	t.Parallel()
+
+	cname := &dns.CNAME{
+		Hdr: dns.RR_Header{
+			Name: "git.eeqj.de.", Rrtype: dns.TypeCNAME, Class: dns.ClassINET,
+		},
+		Target: "fsn1app1.datavi.be.",
+	}
+
+	resp := &resolver.NameserverResponse{Records: map[string][]string{}}
+
+	for _, qtype := range []uint16{
+		dns.TypeA, dns.TypeAAAA, dns.TypeCNAME, dns.TypeMX,
+		dns.TypeTXT, dns.TypeSRV, dns.TypeCAA, dns.TypeNS,
+	} {
+		msg := new(dns.Msg)
+		msg.SetQuestion("git.eeqj.de.", qtype)
+		msg.Answer = []dns.RR{cname}
+
+		resolver.CollectAnswerRecords(msg, resp)
+	}
+
+	assert.Equal(t,
+		map[string][]string{"CNAME": {"fsn1app1.datavi.be."}},
+		resp.Records,
+	)
+}
+
 // TestShuffled shuffles the root servers with many seeds. Every order
 // must hold each root server once, so each is tried before a
 // resolution fails; each root server must come first for some seed, so