MINOR: protocol: always initialize the receivers list on registration

Till now, protocols were required to self-initialize their receivers
list head, which is not very convenient, and is quite error prone.
Indeed, it's too easy to copy-paste a protocol definition and forget
to update the .receivers field to point to itself, resulting in mixed
lists. Let's just do that in protocol_register(). And while we're at
it, let's also zero the nb_receivers entry that works with it, so that
the protocol definition isn't required to pre-initialize stuff related
to internal book-keeping.

src/protocol.c

@@ -44,6 +44,9 @@ void protocol_register(struct protocol *proto)
 	BUG_ON(sock_family < 0 || sock_family >= AF_CUST_MAX);
 	BUG_ON(proto->proto_type >= PROTO_NUM_TYPES);
 
+	LIST_INIT(&proto->receivers);
+	proto->nb_receivers = 0;
+
 	HA_SPIN_LOCK(PROTO_LOCK, &proto_lock);
 	LIST_APPEND(&protocols, &proto->list);
 	__protocol_by_family[sock_family]