Pull request: 1868 fix rdns

Merge in DNS/adguard-home from 1868-rdns-ipv6 to master

Updates #2943.
Updates #2704.

Squashed commit of the following:

commit 53d67ecf17ed4f9c544344288b58f3596c7246e2
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Apr 13 16:18:33 2021 +0300

    all: imp code, docs

commit 2bc15941b87f92b6fa0a7568538e02700a4385a3
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Apr 13 16:09:08 2021 +0300

    all: imp code

internal/aghnet/subnetdetector.go

@@ -100,6 +100,7 @@ func NewSubnetDetector() (snd *SubnetDetector, err error) {
 		"::1/128",
 		"fe80::/10",
 		"2001:db8::/32",
+		"fd00::/8",
 	}
 
 	snd = &SubnetDetector{

internal/aghnet/subnetdetector_test.go

@@ -205,6 +205,14 @@ func TestSubnetDetector_DetectLocallyServedNetwork(t *testing.T) {
 		name: "linked-scoped_unicast",
 		ip:   net.ParseIP("fe80::"),
 		want: true,
+	}, {
+		name: "locally_assigned",
+		ip:   net.ParseIP("fd00::1"),
+		want: true,
+	}, {
+		name: "not_locally_assigned",
+		ip:   net.ParseIP("fc00::1"),
+		want: false,
 	}}
 
 	for _, tc := range testCases {