timesync: IPTOS_LOWDELAY --> IPTOS_DSCP_EF

Deprecated IPTOS_LOWDELAY is ignored by most of today's
network equipment that only ever care about DSCP.

Use the DSCP found in other NTP implementations and set the appropiate
TCLASS for IPv6.

src/timesync/timesyncd-manager.c

@@ -658,8 +658,7 @@ static int manager_listen_setup(Manager *m) {
         if (r < 0)
                 return r;
 
-        if (addr.sa.sa_family == AF_INET)
-                (void) setsockopt_int(m->server_socket, IPPROTO_IP, IP_TOS, IPTOS_LOWDELAY);
+        (void) socket_set_option(m->server_socket, addr.sa.sa_family, IP_TOS, IPV6_TCLASS, IPTOS_DSCP_EF);
 
         return sd_event_add_io(m->event, &m->event_receive, m->server_socket, EPOLLIN, manager_receive_response, m);
 }