fix: resolve the issue with DHCP lease not being renewed

Very easily reproduced when you start a node with a Dynamic IP.
Normally it should renew lease after TTL/2, but that doesn't happen, so
the node starts to get next IP one after another.

After looking at packets sent by other clients, found out that they
have `Client IP address` equal to the IP given by the DHCP server.

Additionally, changed DHCP client to send Request packets directly to the DHCP server after getting an offer.
It looks like DHCP spec states that you should use unicast request directly to DHCP server, not broadcast.

Signed-off-by: Artem Chernyshev <artem.0xD2@gmail.com>

internal/app/networkd/pkg/address/dhcp4.go

@@ -207,10 +207,24 @@ func (d *DHCP4) discover(ctx context.Context) error {
 	}
 
 	mods := []dhcpv4.Modifier{dhcpv4.WithRequestedOptions(opts...)}
+	clientOpts := []nclient4.ClientOpt{}
+
+	if d.Offer != nil {
+		// do not use broadcast, but send the packet to DHCP server directly
+		addr, err := net.ResolveUDPAddr("udp", d.Offer.ServerIPAddr.String()+":67")
+		if err != nil {
+			return err
+		}
+
+		// by default it's set to 0.0.0.0 which actually breaks lease renew
+		d.Offer.ClientIPAddr = d.Offer.YourIPAddr
+
+		clientOpts = append(clientOpts, nclient4.WithServerAddr(addr))
+	}
 
 	// TODO expose this ( nclient4.WithDebugLogger() ) with some
 	// debug logging option
-	cli, err := nclient4.New(d.NetIf.Name)
+	cli, err := nclient4.New(d.NetIf.Name, clientOpts...)
 	if err != nil {
 		return err
 	}