proxy: reuse id when server removed it

reset number of fds before we begin reading.
When we destroy the proxy, just clear the id. We then remove the id
completely when the server sends a remove_id request. This avoid
reusing the proxy id before the server has finished with it.
Add some debug

src/modules/module-protocol-native/connection.c

@@ -158,6 +158,7 @@ static bool refill_buffer(struct pw_protocol_native_connection *conn, struct buf
 	buf->buffer_size += len;
 
 	/* handle control messages */
+	buf->n_fds = 0;
 	for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
 		if (cmsg->cmsg_level != SOL_SOCKET || cmsg->cmsg_type != SCM_RIGHTS)
 			continue;