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_udp.c b/src/proto_udp.c
index 82a9387..eea2583 100644
--- a/src/proto_udp.c
+++ b/src/proto_udp.c
@@ -55,7 +55,7 @@
 	.l3_addrlen = 32/8,
 	.accept = NULL,
 	.connect = NULL,
-	.bind = udp_bind_listener,
+	.listen = udp_bind_listener,
 	.enable_all = enable_all_listeners,
 	.get_src = udp_get_src,
 	.get_dst = udp_get_dst,
@@ -79,7 +79,7 @@
 	.l3_addrlen = 128/8,
 	.accept = NULL,
 	.connect = NULL,
-	.bind = udp_bind_listener,
+	.listen = udp_bind_listener,
 	.enable_all = enable_all_listeners,
 	.get_src = udp6_get_src,
 	.get_dst = udp6_get_dst,