Fix ed25519 dropbear to openssh conversion This introduces buf_put_ed25519_priv_ossh and buf_get_ed25519_priv_ossh to handle OpenSSH internal private key format. Previously writing OpenSSH format keys didn't write the private part correctly.

#include "tommath_private.h"
#ifdef BN_MP_SET_I64_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */

MP_SET_SIGNED(mp_set_i64, mp_set_u64, int64_t, uint64_t)
#endif