Replace ChanType.sepfds with Channel.bidir_fd This handles the case where a svrchansess has separate FDs for nopty, but a single FD for pty mode. The use of sepfds was also previously incorrect for X11 and agent forwarding

#include "tommath_private.h"
#ifdef BN_MP_INIT_U32_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */

MP_INIT_INT(mp_init_u32, mp_set_u32, uint32_t)
#endif