sshpkt_put
(r = sshpkt_put(ssh, cp, have)) != 0 ||
int sshpkt_put(struct ssh *ssh, const void *v, size_t len);
(r = sshpkt_put(ssh, data, len)) != 0 ||
(r = sshpkt_put(ssh, mech->elements, mech->length)) != 0 ||