kex_prop2buf
if ((r = kex_prop2buf(ssh->kex->my, prop)) != 0) {
if ((r = kex_prop2buf(ssh->kex->my, proposal)) != 0)
int kex_prop2buf(struct sshbuf *, char *proposal[PROPOSAL_MAX]);
r = kex_prop2buf(ssh->kex->my, proposal);