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 unchanged. 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

README.md

@@ -63,8 +63,8 @@ rejected.
   merged view — it is a map from nameserver to record set.
 - DNS names inside record values (CNAME, MX, SRV and NS targets) are
   stored in lower case, because names are case-insensitive and
-  nameservers may answer in any letter case. TXT and CAA values keep
-  their letter case; they are not lower-cased.
+  nameservers may answer in any letter case. TXT and CAA values are
+  stored exactly as received.
 - Any observable change in any nameserver's response triggers a
   notification. This includes:
   - **Record change**: A nameserver returns different records than it

internal/resolver/iterative.go

@@ -610,8 +610,8 @@ func classifyResponse(resp *NameserverResponse, state queryState) {
 
 // extractRecordValue formats a DNS RR value as a string. DNS names
 // are case-insensitive and nameservers may answer in any letter case,
-// so names are lower-cased to compare equal. TXT and CAA values keep
-// their letter case.
+// so names are lower-cased to compare equal. TXT and CAA values are
+// kept exactly as received.
 func extractRecordValue(rr dns.RR) string {
 	switch r := rr.(type) {
 	case *dns.A: