packet: set SO_REUSEADDR on babel port

This allows to listen to incoming babel packets along side another babel
instance.

packet/packet.go

@@ -1,8 +1,10 @@
 package packet
 
 import (
+	"context"
 	"fmt"
 	"net"
+	"syscall"
 
 	"golang.org/x/net/ipv6"
 	"inet.af/netaddr"
@@ -34,10 +36,21 @@ func Listen(group string, port uint16, ifaces ...string) (Conn, error) {
 func ListenPort(port uint16) (Conn, error) {
 	var c Conn
 
-	uc, err := net.ListenUDP("udp6", &net.UDPAddr{Port: int(port)})
+	lc := net.ListenConfig{
+		Control: func(network, address string, c syscall.RawConn) error {
+			var err error
+			c.Control(func(fd uintptr) {
+				err = syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1)
+			})
+			return err
+		},
+	}
+	uc, err := lc.ListenPacket(context.Background(), "udp6", fmt.Sprintf(":%d", port))
+
 	if err != nil {
 		return c, err
 	}
+
 	c.v6pc = ipv6.NewPacketConn(uc)
 
 	return c, c.v6pc.SetControlMessage(