MINOR: protocol: rename the ->bind field to ->listen

The function currently is doing both the bind() and the listen(), so
let's call it ->listen so that the bind() operation can move to another
place.
diff --git a/src/proto_tcp.c b/src/proto_tcp.c
index 258a72d..a400b0f 100644
--- a/src/proto_tcp.c
+++ b/src/proto_tcp.c
@@ -59,7 +59,7 @@
 	.l3_addrlen = 32/8,
 	.accept = &listener_accept,
 	.connect = tcp_connect_server,
-	.bind = tcp_bind_listener,
+	.listen = tcp_bind_listener,
 	.enable_all = enable_all_listeners,
 	.get_src = sock_get_src,
 	.get_dst = sock_inet_get_dst,
@@ -83,7 +83,7 @@
 	.l3_addrlen = 128/8,
 	.accept = &listener_accept,
 	.connect = tcp_connect_server,
-	.bind = tcp_bind_listener,
+	.listen = tcp_bind_listener,
 	.enable_all = enable_all_listeners,
 	.get_src = sock_get_src,
 	.get_dst = sock_get_dst,