[tcp] Add AF_INET6 socket opener

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

src/net/tcp.c

  ***************************************************************************
  */
 
-/** TCP socket opener */
-struct socket_opener tcp_socket_opener __socket_opener = {
+/** TCP IPv4 socket opener */
+struct socket_opener tcp_ipv4_socket_opener __socket_opener = {
 	.semantics	= TCP_SOCK_STREAM,
 	.family		= AF_INET,
 	.open		= tcp_open,
 };
 
+/** TCP IPv6 socket opener */
+struct socket_opener tcp_ipv6_socket_opener __socket_opener = {
+	.semantics	= TCP_SOCK_STREAM,
+	.family		= AF_INET6,
+	.open		= tcp_open,
+};
+
 /** Linkage hack */
 int tcp_sock_stream = TCP_SOCK_STREAM;