Pull request: 3381 check private domains

Merge in DNS/adguard-home from 3381-validate-privateness to master

Closes #3381.

Squashed commit of the following:

commit 21cb12d10b07bb0bf0578db74ca9ac7b3ac5ae14
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Feb 14 16:29:59 2022 +0300

    all: imp code, docs

commit 39793551438cbea71e6ec78d0e05bee2d8dba3e5
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Feb 14 15:08:36 2022 +0300

    all: imp code, docs

commit 6b71848fd0980582b1bfe24a34f48608795e9b7d
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Feb 14 14:22:00 2022 +0300

    all: check private domains

internal/dnsforward/testdata/TestDNSForwardHTTP_handleSetConfig.json

@@ -520,6 +520,43 @@
       ]
     }
   },
+  "local_ptr_upstreams_bad": {
+    "req": {
+      "local_ptr_upstreams": [
+        "123.123.123.123",
+        "[/non.arpa/]#"
+      ]
+    },
+    "want": {
+      "upstream_dns": [
+        "8.8.8.8:53",
+        "8.8.4.4:53"
+      ],
+      "upstream_dns_file": "",
+      "bootstrap_dns": [
+        "9.9.9.10",
+        "149.112.112.10",
+        "2620:fe::10",
+        "2620:fe::fe:10"
+      ],
+      "protection_enabled": true,
+      "ratelimit": 0,
+      "blocking_mode": "",
+      "blocking_ipv4": "",
+      "blocking_ipv6": "",
+      "edns_cs_enabled": false,
+      "dnssec_enabled": false,
+      "disable_ipv6": false,
+      "upstream_mode": "",
+      "cache_size": 0,
+      "cache_ttl_min": 0,
+      "cache_ttl_max": 0,
+      "cache_optimistic": false,
+      "resolve_clients": false,
+      "use_private_ptr_resolvers": false,
+      "local_ptr_upstreams": []
+    }
+  },
   "local_ptr_upstreams_null": {
     "req": {
       "local_ptr_upstreams": null