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.go

@@ -0,0 +1,58 @@
+package dhcpd
+
+import (
+	"bytes"
+	"fmt"
+)
+
+// nullBool is a nullable boolean.  Use these in JSON requests and responses
+// instead of pointers to bool.
+//
+// TODO(a.garipov): Inspect uses of *bool, move this type into some new package
+// if we need it somewhere else.
+type nullBool uint8
+
+// nullBool values
+const (
+	nbNull nullBool = iota
+	nbTrue
+	nbFalse
+)
+
+// String implements the fmt.Stringer interface for nullBool.
+func (nb nullBool) String() (s string) {
+	switch nb {
+	case nbNull:
+		return "null"
+	case nbTrue:
+		return "true"
+	case nbFalse:
+		return "false"
+	}
+
+	return fmt.Sprintf("!invalid nullBool %d", uint8(nb))
+}
+
+// boolToNullBool converts a bool into a nullBool.
+func boolToNullBool(cond bool) (nb nullBool) {
+	if cond {
+		return nbTrue
+	}
+
+	return nbFalse
+}
+
+// UnmarshalJSON implements the json.Unmarshaler interface for *nullBool.
+func (nb *nullBool) UnmarshalJSON(b []byte) (err error) {
+	if len(b) == 0 || bytes.Equal(b, []byte("null")) {
+		*nb = nbNull
+	} else if bytes.Equal(b, []byte("true")) {
+		*nb = nbTrue
+	} else if bytes.Equal(b, []byte("false")) {
+		*nb = nbFalse
+	} else {
+		return fmt.Errorf("invalid nullBool value %q", b)
+	}
+
+	return nil
+}