Pull request 2256: 4923 Better interfaces

Updates #4923.

Squashed commit of the following:

commit 0e40b41aa1e517a62d6076c4e7a57c607792ef01
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Jul 10 15:28:16 2024 +0300

    dhcpsvc: imp code, docs

commit 5463fdde473f84caaca229b53027e8183d5c6bdc
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Jul 9 20:31:20 2024 +0300

    dhcpsvc: imp ifaces

internal/dhcpsvc/lease.go

@@ -1,7 +1,6 @@
 package dhcpsvc
 
 import (
-	"bytes"
 	"net"
 	"net/netip"
 	"slices"
@@ -45,8 +44,3 @@ func (l *Lease) Clone() (clone *Lease) {
 		IsStatic: l.IsStatic,
 	}
 }
-
-// compareLeaseMAC compares two [Lease]s by hardware address.
-func compareLeaseMAC(a, b *Lease) (res int) {
-	return bytes.Compare(a.HWAddr, b.HWAddr)
-}