MINOR: protocol: add a new ->bind() entry to bind the receiver

This will be the function that must be used to bind the receiver. It
solely depends on the address family but for now it's simpler to have
it per protocol.
diff --git a/include/haproxy/protocol-t.h b/include/haproxy/protocol-t.h
index 07532e1..f29c944 100644
--- a/include/haproxy/protocol-t.h
+++ b/include/haproxy/protocol-t.h
@@ -30,6 +30,7 @@
 
 /* some pointer types referenced below */
 struct listener;
+struct receiver;
 struct connection;
 
 /*
@@ -71,6 +72,7 @@
 	socklen_t sock_addrlen;				/* socket address length, used by bind() */
 	int l3_addrlen;					/* layer3 address length, used by hashes */
 	void (*accept)(int fd);				/* generic accept function */
+	int (*bind)(struct receiver *rx, void (*handler)(int fd), char **errmsg); /* bind a receiver */
 	int (*listen)(struct listener *l, char *errmsg, int errlen); /* start a listener */
 	int (*enable_all)(struct protocol *proto);	/* enable all bound listeners */
 	int (*disable_all)(struct protocol *proto);	/* disable all bound listeners */