Pull request: dnsforward: fix clientid check

Closes #3437.

Squashed commit of the following:

commit fc4207a6ee1a09ade9db5eb5c8b58f88011db2f9
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Aug 12 18:22:31 2021 +0300

    dnsforward: imp code, docs

commit 0c608e0b7ca0b68b7810fc1ca798fb7d80d6ac24
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Aug 12 18:01:22 2021 +0300

    dnsforward: fix clientid check

internal/dnsforward/clientid_test.go

@@ -134,6 +134,15 @@ func TestServer_clientIDFromDNSContext(t *testing.T) {
 		wantClientID: "cli",
 		wantErrMsg:   "",
 		strictSNI:    true,
+	}, {
+		name:         "tls_client_id_issue3437",
+		proto:        proxy.ProtoTLS,
+		hostSrvName:  "example.com",
+		cliSrvName:   "cli.myexample.com",
+		wantClientID: "",
+		wantErrMsg: `client id check: client server name "cli.myexample.com" ` +
+			`doesn't match host server name "example.com"`,
+		strictSNI: true,
 	}}
 
 	for _, tc := range testCases {