Pull request: dhcpd: do not override ra-slaac settings

Merge in DNS/adguard-home from 2653-ra-slaac to master

Updates #2653.

Squashed commit of the following:

commit f261413a58dc813e37cc848606ed490b8c0ac9f3
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Feb 11 20:37:13 2021 +0300

    all: doc changes, rm debug

commit 4a8c6e4897579493c1ca242fb8f0f440c3b51a74
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Feb 11 20:11:46 2021 +0300

    dhcpd: do not override ra-slaac settings

internal/dhcpd/nullbool_test.go

@@ -0,0 +1,69 @@
+package dhcpd
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNullBool_UnmarshalText(t *testing.T) {
+	testCases := []struct {
+		name       string
+		data       []byte
+		wantErrMsg string
+		want       nullBool
+	}{{
+		name:       "empty",
+		data:       []byte{},
+		wantErrMsg: "",
+		want:       nbNull,
+	}, {
+		name:       "null",
+		data:       []byte("null"),
+		wantErrMsg: "",
+		want:       nbNull,
+	}, {
+		name:       "true",
+		data:       []byte("true"),
+		wantErrMsg: "",
+		want:       nbTrue,
+	}, {
+		name:       "false",
+		data:       []byte("false"),
+		wantErrMsg: "",
+		want:       nbFalse,
+	}, {
+		name:       "invalid",
+		data:       []byte("flase"),
+		wantErrMsg: `invalid nullBool value "flase"`,
+		want:       nbNull,
+	}}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			var got nullBool
+			err := got.UnmarshalJSON(tc.data)
+			if tc.wantErrMsg == "" {
+				assert.Nil(t, err)
+			} else {
+				require.NotNil(t, err)
+				assert.Equal(t, tc.wantErrMsg, err.Error())
+			}
+
+			assert.Equal(t, tc.want, got)
+		})
+	}
+
+	t.Run("json", func(t *testing.T) {
+		want := nbTrue
+		var got struct {
+			A nullBool
+		}
+
+		err := json.Unmarshal([]byte(`{"A":true}`), &got)
+		require.Nil(t, err)
+		assert.Equal(t, want, got.A)
+	})
+}