MINOR: protocol: make proto_tcp & proto_uxst report listening sockets

Now we introdce a new .rx_listening() function to report if a receiver is
actually a listening socket. The reason for this is to help detect shared
sockets that might have been broken by sibling processes.

include/haproxy/protocol-t.h

@@ -101,6 +101,7 @@ struct protocol {
 	void (*rx_unbind)(struct receiver *rx);         /* unbind the receiver, most often closing the FD */
 	int (*rx_suspend)(struct receiver *rx);         /* temporarily suspend this receiver for a soft restart */
 	int (*rx_resume)(struct receiver *rx);          /* try to resume a temporarily suspended receiver */
+	int (*rx_listening)(const struct receiver *rx); /* is the receiver listening ? 0=no, >0=OK, <0=unrecoverable */
 
 	/* functions acting on connections */
 	void (*accept)(int fd);				/* generic accept function */

src/proto_tcp.c

@@ -72,6 +72,7 @@ static struct protocol proto_tcpv4 = {
 	.rx_unbind = sock_unbind,
 	.rx_suspend = tcp_suspend_receiver,
 	.rx_resume = tcp_resume_receiver,
+	.rx_listening = sock_accept_conn,
 	.accept = &listener_accept,
 	.connect = tcp_connect_server,
 	.receivers = LIST_HEAD_INIT(proto_tcpv4.receivers),
@@ -100,6 +101,7 @@ static struct protocol proto_tcpv6 = {
 	.rx_unbind = sock_unbind,
 	.rx_suspend = tcp_suspend_receiver,
 	.rx_resume = tcp_resume_receiver,
+	.rx_listening = sock_accept_conn,
 	.accept = &listener_accept,
 	.connect = tcp_connect_server,
 	.receivers = LIST_HEAD_INIT(proto_tcpv6.receivers),

src/proto_uxst.c

@@ -65,6 +65,7 @@ static struct protocol proto_unix = {
 	.rx_disable = sock_disable,
 	.rx_unbind = sock_unbind,
 	.rx_suspend = uxst_suspend_receiver,
+	.rx_listening = sock_accept_conn,
 	.accept = &listener_accept,
 	.connect = &uxst_connect_server,
 	.receivers = LIST_HEAD_INIT(proto_unix.receivers),