resolver: lower-case DNS names in record values (closes #157)

Nameservers may answer with DNS names in any letter case. For eeqj.de,
y.ns.joker.com answers in upper case while its peers answer in lower
case, so the inconsistency check fired on every cycle. extractRecordValue
now lower-cases CNAME, MX, SRV and NS targets, so the inconsistency check
and the record-change check both compare names regardless of case. A,
AAAA, TXT and CAA values are formatted as before. State saved before this
change can hold upper-case names, which report a one-time record change
on the first check after upgrading.

Model: opus-5-5

internal/resolver/export_test.go

@@ -0,0 +1,8 @@
+package resolver
+
+import "github.com/miekg/dns"
+
+// ExtractRecordValue exports extractRecordValue for testing.
+func ExtractRecordValue(rr dns.RR) string {
+	return extractRecordValue(rr)
+}