Pull request 1943: 6046 Local PTR

Merge in DNS/adguard-home from 6046-local-ptr to master

Updates #6046.

Squashed commit of the following:

commit 3e90815f29173d2f68970278bd7b1b29cc0a4465
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Jul 27 18:17:41 2023 +0300

    all: log changes

commit 7639f6f785670c15911fb3ca20abeb4e2b8f8582
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Jul 27 17:40:49 2023 +0300

    all: fix 0 ttl ptr

internal/rdns/rdns.go

@@ -101,6 +101,8 @@ func (r *Default) Process(ip netip.Addr) (host string, changed bool) {
 		log.Debug("rdns: cache: adding item %q: %s", ip, err)
 	}
 
+	// TODO(e.burkov):  The name doesn't change if it's neither stored in cache
+	// nor resolved successfully.  Is it correct?
 	return host, fromCache == "" || host != fromCache
 }