[udp] Add AF_INET6 socket opener

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/udp.c

  ***************************************************************************
  */
 
-/** UDP socket opener */
-struct socket_opener udp_socket_opener __socket_opener = {
+/** UDP IPv4 socket opener */
+struct socket_opener udp_ipv4_socket_opener __socket_opener = {
 	.semantics	= UDP_SOCK_DGRAM,
 	.family		= AF_INET,
 	.open		= udp_open,
 };
 
+/** UDP IPv6 socket opener */
+struct socket_opener udp_ipv6_socket_opener __socket_opener = {
+	.semantics	= UDP_SOCK_DGRAM,
+	.family		= AF_INET6,
+	.open		= udp_open,
+};
+
 /** Linkage hack */
 int udp_sock_dgram = UDP_SOCK_DGRAM;