Pull request: 4939 Client update

Merge in DNS/adguard-home from 4939-client-upd to master

Updates #4939.

Squashed commit of the following:

commit 34f35822afcc8020a674cd023a5907b5b3edcb65
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Feb 10 14:01:57 2023 +0300

    all: imp code, docs

commit 1cd8767a38f6494c92fb5ceff26abe228fcca638
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Feb 9 17:20:56 2023 +0300

    all: different ttls

commit 66d951ba3dd72cb698b89b432cbbbdd65cb421a2
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Feb 9 14:24:47 2023 +0300

    all: imp code

commit 3fb8d08310296dad90783f13ba46a1d0ea11da2e
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Feb 8 19:35:29 2023 +0300

    home: fix rdns check logic

internal/dnsforward/dnsforward_test.go

@@ -1221,6 +1221,9 @@ func TestServer_Exchange(t *testing.T) {
 
 	errUpstream := aghtest.NewErrorUpstream()
 	nonPtrUpstream := aghtest.NewBlockUpstream("some-host", true)
+	refusingUpstream := aghtest.NewUpstreamMock(func(req *dns.Msg) (resp *dns.Msg, err error) {
+		return new(dns.Msg).SetRcode(req, dns.RcodeRefused), nil
+	})
 
 	srv := &Server{
 		recDetector: newRecursionDetector(0, 1),
@@ -1265,15 +1268,21 @@ func TestServer_Exchange(t *testing.T) {
 	}, {
 		name:        "empty_answer_error",
 		want:        "",
-		wantErr:     rDNSEmptyAnswerErr,
+		wantErr:     ErrRDNSNoData,
 		locUpstream: locUpstream,
 		req:         net.IP{192, 168, 1, 2},
 	}, {
-		name:        "not_ptr_error",
+		name:        "invalid_answer",
 		want:        "",
-		wantErr:     rDNSNotPTRErr,
+		wantErr:     ErrRDNSNoData,
 		locUpstream: nonPtrUpstream,
 		req:         localIP,
+	}, {
+		name:        "refused",
+		want:        "",
+		wantErr:     ErrRDNSFailed,
+		locUpstream: refusingUpstream,
+		req:         localIP,
 	}}
 
 	for _, tc := range testCases {